Chapter 5: Document Structure

5.5. The ‘symbol’ element

The ‘symbol’ element is used to define graphical templates which can be instantiated by a ‘use’ element but which are not rendered directly.

A ‘symbol’ establishes a nested coordinate system for the graphics it contains. When a symbol is instantiated as the referenced element of a ‘use’ element, it is therefore rendered very similarly to a nested ‘svg’ element.

The x, y, width, and height geometry properties have the same effect as on an ‘svg’ element, when the ‘symbol’ is instantiated by a ‘use’ element. In particular, if width and height compute to auto (and are not over-ridden by values on the instantiating ‘use’ element), then they will be treated as a value of 100%.

New in SVG 2. Allowing geometry properties to be specified on a symbol provides a more consistent rendering model, and allows authors to set a default size for each symbol (which may still be over-ridden by attributes on the ‘use’ element).

5.5.1. Attributes

Name	Value	Initial value	Animatable
refX	<length> | left | center | right	(none)	yes
refY	<length> | top | center | bottom	(none)	yes

New in SVG 2. Added to make it easier to align symbols to a particular point, as is often done in maps. Similar to the matching attributes on ‘marker’.

Add refX/refY to symbol element. Resolved at Leipzig F2F. Status: Done.

We will add top/center/bottom, left/center/right keywords to refX/refY on marker/symbol. Resolved at London F2F. Values inspired by 'background-position'. Status: Done.

The ‘refX’ and ‘refY’ attributes define the reference point of the symbol which is to be placed exactly at the symbol's x,y positioning coordinate, as defined by the cumulative effect of the x and y properties and any transformations on the ‘symbol’ and its host ‘use’ element.

Keyword values have the same meaning as for the ‘refX’ and ‘refY’ attributes on the ‘marker’ element, resolving to 0%, 50%, or 100% in the applicable direction.

Unlike other positioning attributes, ‘refX’ and ‘refY’ are interpreted as being in the coordinate system of the symbol contents, after application of the ‘viewBox’ and ‘preserveAspectRatio’ attributes. If one or both of the attributes is not specified, no adjustment is made in the corresponding dimension, and the top or left side of the symbol's rectangular viewport region (regardless of the ‘viewBox’ coordinates) is positioned at the x,y point.

For backwards compatibility, the behavior when ‘refX’ and ‘refY’ are not specified on a ‘symbol’ is different from when they are specified with a value of 0, and therefore different from the behavior when equivalent attributes are not specified on a ‘marker’.

5.5.2. Notes on symbols

The use of ‘symbol’ elements for graphics that are used multiple times in the same document adds structure and semantics. Closely related to the ‘symbol’ element are the ‘marker’ and ‘pattern’ elements; all three define a container of graphical content that can be rendered repeatedly at various positions and scales in the SVG. However, while re-used graphics in a pattern and marker provide a graphical effect on another element, the content in a ‘symbol’ will be embedded as fully interactive content, within a use-element shadow tree.

The user agent style sheet sets the overflow property for ‘symbol’ elements to hidden, which causes a rectangular clipping path to be created at the bounds of symbol's SVG viewport. Unless the overflow property is overridden, any graphics within the symbol which goes outside of the symbol's SVG viewport will be clipped.

‘symbol’ elements must never be rendered directly; their only usage is as something that can be referenced using the ‘use’ element. The user agent must set the display property on the ‘symbol’ element to none, as part of the user agent style sheet, and this declaration must have importance over any other CSS rule or presentation attribute.

The generated instance of a ‘symbol’ that is the direct referenced element of a ‘use’ element must always have a computed value of inline for the display property. In other words, it must be rendered whenever the host ‘use’ element is rendered. The user agent style sheet again defines this declaration to have importance over any other CSS rule or presentation attribute. Any other ‘symbol’ that is cloned to create an element instance within the use-element shadow tree behaves as a symbol definition, and must not be rendered.

5.6. The ‘use’ element

The ‘use’ element references another element, a copy of which is rendered in place of the ‘use’ in the document. The referenced element may be a container element, in which case a copy of the complete SVG document subtree rooted at that element is used.

The cloned content inherits styles from the ‘use’ element and can be the target of user events. However, these cloned element instances remain linked to the referenced source and reflect DOM mutations in the original. In addition, all style rules that apply in the scope of the referenced element also apply in the scope of the cloned shadow tree.

The x, y, width and height geometric properties specify the positioning of the referenced element. The width and height attributes only have an effect if the referenced element defines a viewport (i.e., if it is a ‘svg’ or ‘symbol’); if so, a value other than auto for the ‘use’ element overrides the value of the corresponding geometric property on that element.

A negative value for width or height must be treated as an illegal value. If width or height is zero, and the properties have an effect on the referenced element, then rendering of that element will be disabled.

The x and y properties affect the user coordinate system for the element. See the Layout section for implementation details.

Name	Value	Initial value	Animatable
href	URL [URL]	(none)	yes

An URL reference to the element/fragment within an SVG document to be cloned for rendering.

The ‘use’ element can reference an entire SVG document by specifying an ‘href’ value without a fragment. Such references are taken to be referring to the root element of the referenced document.

Refer to the common handling defined for URL reference attributes and deprecated XLink attributes.

New in SVG 2. An ‘href’ without a fragment allows an entire SVG document to be referenced without having to ensure that it has an ID on its root element.

User agents may restrict external resource documents for security reasons. In particular, this specification does not allow cross-origin resource requests in ‘use’. A future version of this or another specification may provide a method of securely enabling cross-origin re-use of assets.

When the ‘href’ attribute is set (or, in the absence of an ‘href’ attribute, an ‘xlink:href’ attribute), the user agent must process the URL. The target element that results from URL processing is the referenced element of the ‘use’.

If the referenced element that results from resolving the URL is neither an SVG element nor an HTML-namespaced element that may be included as a child of an SVG container element, then the reference is invalid and the ‘use’ element is in error.

If the referenced element is a (shadow-including) ancestor of the ‘use’ element, then this is an invalid circular reference and the ‘use’ element is in error.

Otherwise, the user agent must generate a shadow tree of re-used graphics to render as the contents of the ‘use’ element, as described in the next section, The use-element shadow tree.

A ‘use’ that has an unresolved or invalid URL reference is not rendered. For the purpose of bounding box calculations, it is equivalent to an empty container element.

5.6.1. The use-element shadow tree

The re-used graphics generated by a ‘use’ element are defined in terms of a shadow tree. In terms of interactivity and style inheritance, they are therefore quite different from other types of re-used graphics in SVG, such as ‘pattern’ and ‘marker’ content.

Elements in the shadow tree are rendered as if the ‘use’ element was a container and they were its children. However, the SVG Document Object Model (DOM) only contains the ‘use’ element and its attributes. The SVG DOM does not include the element instances as children of the ‘use’ element.

User agents that support scripting and the document object model must implement the use-element shadow tree as described in this section and in conformance with the dom specification [dom], or its future replacement. In contrast, user agents that do not support the dynamic interactive processing mode may not need to implement all the details of the shadow DOM. However, all user agents must ensure that the layout and style inheritance for the re-used graphics, and any multimedia and declarative animations if applicable, are rendered in the same way as if the shadow DOM was implemented.

The following definitions apply when discussing ‘use’ elements and their shadow trees:

referenced element

The element specified by the ‘href’ (or ‘xlink:href’) attribute on the ‘use’ element, or the root element of a document referenced by that attribute if the URL provided does not include a target fragment that links to a specific element ‘id’.

referenced document subtree

referenced graphics

The referenced element, and all of its descendent nodes.

shadow root

A ShadowRoot object, a type of DocumentFragment node which is associated with a host Element, and which contains the content that will be used to render that host. A shadow root should be implemented in conformance with the dom specification [dom], or its future replacement.

shadow host

host

An element that has an associated shadow root; usage is consistent the definition of host in the DOM standard.

shadow tree

A node tree whose root is a shadow root; usage is consistent the definition of shadow tree in the DOM standard.

use-element shadow tree

A shadow tree whose host is a ‘use’ element, which contains element instances generated by cloning the referenced graphics.

element instance

instance

An element in the use-element shadow tree, which is generated by cloning a corresponding element in the referenced document subtree.

instance root

The element instance for the referenced element; it is always a direct child of the ‘use’ element's shadow root.

corresponding element

For each element instance, the element in the referenced document subtree from which it is cloned.

corresponding use element

For each element instance, the ‘use’ element which causes it to be rendered in the document. This is the instance's shadow root's host ‘use’ element if that element is not itself an element instance within a ‘use’ element shadow tree, or is that element's corresponding use element otherwise, recursively exiting shadow trees as many times as necessary to reach a ‘use’ element that was not itself generated as part of the shadow tree of another ‘use’ element.

When the user agent successfully resolves a ‘use’ element to identify a referenced element, the user agent must create a use-element shadow tree whose host is the ‘use’ element itself. The shadow tree must be created even if the ‘use’ element is not rendered because it is a descendent of a never-rendered element, because of conditional processing, or because of the display property being set to none on it or an ancestor element.

Each node in the shadow tree is an instance of a corresponding node from the referenced document subtree. The shadow nodes all descend from the instance root, which is the instance of the referenced element, and which itself is a direct child of the shadow root node.

The shadow tree is open (inspectable by script), but read-only. Any attempt to directly modify the elements, attributes, and other nodes in the shadow tree must throw a NoModificationAllowedError.

Within a use-element shadow tree, ‘script’ elements are inert (do not execute); ‘audio’ and ‘video’ elements are subject to the limitations specified in the Multimedia section.

Previous versions of SVG restricted the contents of the shadow tree to SVG graphics elements. This specification allows any valid SVG document subtree to be cloned. Cloning non-graphical content, however, will not usually have any visible effect.

If the referenced element is in an external file, then all URL references in attributes and style properties must be made absolute as described in Generating the absolute URL, before copying the value to the element instances. The shadow tree itself uses the same document base URL as the document that includes it.

The user agent must ensure that all mutations to the referenced document subtree are reflected in the shadow tree. This includes changes to elements, attributes, and text and other nodes. In addition, changes to the stylesheets in effect for the referenced graphics must be reflected in changes to the stylesheets in the shadow tree's scope, as described futher in the section on style inheritance.

If either the ‘use’ element or the referenced element is altered in a way that causes the ‘use’ element's URL reference to become unresolved again, then the entire shadow tree for that use element is discarded.

When a ‘use’ references another element which is another ‘use’ or whose content contains a ‘use’ element, then the shadow DOM cloning approach described above is recursive. However, a set of references that directly or indirectly reference a element to create a circular dependency is an invalid circular reference. The ‘use’ element or element instance whose shadow tree would create the circular reference is in error and must not be rendered by the user agent.

5.6.2. Layout of re-used graphics

The value of the x, y, width and height properties on a ‘use’ element are used to position the re-used graphics and to set the viewport size if the referenced element defines a nested viewport. The effect of these properties on a ‘use’ element is notably different from their effect on a graphics element, or from their effect in CSS box layout.

The x and y properties define an additional transformation (translate(x,y), where x and y represent the computed value of the corresponding property) to be applied to the ‘use’ element, after any transformations specified with other properties (i.e., appended to the right-side of the transformation list).

The width and height properties on the ‘use’ element override the values for the corresponding properties on a referenced ‘svg’ or ‘symbol’ element when determining the used value for that property on the instance root element. However, if the computed value for the property on the ‘use’ element is auto, then the property is computed as normal for the element instance.

These properties can therefore be used to scale a graphic that defines its own coordinate system, each time it is re-used. Because auto is the initial value, if dimensions are not explicitly set on the ‘use’ element, the values set on the ‘svg’ or ‘symbol’ will be used as defaults.

The width and height properties on the ‘use’ element have no effect if the referenced element does not establish a new viewport. In particular, the ‘use’ element does not itself establish a new viewport, and therefore does not affect the interpretation of percentages in the re-used graphics.

In all other ways, rendering and layout of elements within the use-element shadow tree occurs as if the ‘use’ element was a container for its shadow content. In particular, unless elements within the shadow tree establish a new viewport, they must be drawn in the coordinate system in which the ‘use’ element is defined (including any cumulative transformations). This affects the interpretation of percentage lengths, and also graphical effects with userSpaceOnUse units.

5.6.3. Style Scoping and Inheritance

The use-element shadow tree, like other shadow trees, exhibits style encapsulation, as defined in the CSS Scoping module [css-scoping-1]. This means that elements in the shadow tree inherit styles from its host ‘use’ element, but that style rules defined in the outer document do not match the elements in the shadow tree. Instead, the shadow tree maintains its own list of stylesheets, whose CSS rules are matched against elements in the shadow tree.

Presentation attributes and the ‘style’ attribute are cloned from the elements in the referenced graphics into the element instances in the same manner as other attributes.

When the referenced element is from the same document as the ‘use’ element, the same document stylesheets will apply in both the original document and the shadow tree document fragment. Any changes to the stylesheets in the main document also affect the shadow tree; the StyleSheetList object accessed through the document and shadow root document fragment's styleSheets properties must be identical. If a ‘style’ element is duplicated as part of the referenced document subtree, then the styleSheet property on the element instance points to the same object as for the corresponding element.

When the referenced element is from an external document, the stylesheet objects generated when processing that document apply to the shadow tree, and are read-only. All URL references in the stylesheet, including fragment-only references, must be made absolute, relative to the URL of the document that contains the referenced element. User agents may re-use the same stylesheet objects for any shadow trees that reference that same external document.

Style rules that are scoped to the shadow tree cannot normally affect any elements in the main document. Similarly, style rules in the main document can only affect the shadow tree elements by changing inherited values. However, CSS Scoping defines special selectors for styling the host element from within the shadow tree, or for adjusting styles within the shadow tree in response to changes in the host's context [css-scoping-1].

CSS media queries within a shadow tree's scope are evaluated using the same device features and dimensions as the corresponding "light" document (that is, the document that contains the corresponding use element for the shadow tree, after recursively exiting all nested shadow trees).

In most cases, the element instance in the shadow tree will match the same style rules as its corresponding element in the original document. However, if a CSS rule uses a complex selector to match an element based on its ancestors or siblings, and those ancestors or siblings are not cloned as part of the shadow tree, then that rule would no longer match the element instance. Similarly, child-indexed pseudo-classes such as nth-of-type and nth-child may apply to one element but not the other. This represents a change from how style cloning was defined in previous versions of SVG.

The following example demonstrates both the consistent and changed style-matching rules. The circle on the left is re-used to draw the circle on the right. The original circle has styles set in various ways:

• stroke-width (20) is set in a presentation attribute on the circle itself.
• stroke-opacity (0.7) is set via a CSS rule with a simple selector matching the circle tag name.
• stroke color (green) is set using a complex CSS selector, matching the circle as a descendent of an element with class special.
• fill color is not set directly on the circle, so is inherited from the style set on the containing ‘g’ element (blue).

In the SVG 1.1 style-cloning model, the specified style values would be cloned from the original element to the element instance. The re-used circle would have the same styles as the original, except that the fill value would be inherited from the ‘use’ (orange) instead of from the ‘g’ (blue).

In the shadow DOM model required by SVG 2, the styles for the re-used circle are calculated as follows:

• the stroke-width (20) presentation attribute is cloned to the element instance.
• the CSS rule setting stroke-opacity (0.7) is part of the CSS stylesheet cloned into the shadow tree; it matches the circle tag name of the element instance, so is applied.
• the CSS rule with the complex selector is also part of the cloned stylesheet, but it does not match the element instance of the circle, which is not a descendent of an element with class special; instead, stroke color on the circle is inherited from the host ‘use’ element (purple).
• fill color is still not set directly, so is once again inherited from the host ‘use’ element.

The re-used circle therefore differs from the original in both fill color (because it inherits from a different element) and stroke color (because the complex selector no longer matches).

<svg xmlns="http://www.w3.org/2000/svg"
     xmlns:xlink="http://www.w3.org/1999/xlink"
     width="200" height="100" viewBox="0 0 200 100">
  <title>Style inheritance and the use element</title>
  <desc>
    Two circles, one of which is a re-styled clone of the other.
    This file demonstrates one of the cases where
    the shadow-DOM style matching rules in SVG 2
    have a different effect than the SVG 1.1 style cloning rules.
    The original circle on the left
    should have blue fill
    and green stroke.
    In a conforming SVG 1.1 user agent,
    the re-used circle on the right
    should have orange fill and green stroke.
    In a conforming SVG 2 user agent,
    the re-used circle should have orange fill and purple stroke.
    In all cases,
    the stroke should be partially transparent
    and 20 units wide,
    relative to a total circle diameter of 100 units.
  </desc>
  <style type="text/css">
    circle          { stroke-opacity: 0.7; }
    .special circle { stroke: green; }
    use             { stroke: purple;
                      fill: orange; }
  </style>
  <g class="special" style="fill: blue">
     <circle id="c" cy="50" cx="50" r="40"
             stroke-width="20" />
  </g>
  <use xlink:href="#c" x="100" />
</svg>

Example Use-changed-styles

View this example as SVG (SVG-enabled browsers only)

Previous versions of SVG were not clear about how dynamic pseudo-classes (such as :hover) should apply to element instances. The shadow tree model requires that all such pseudo-classes are matched independently to the element instance or to its corresponding element, depending on which element the user is interacting with.

Specifying 'visibility:hidden' on a ‘use’ element does not guarantee that the referenced content will not be rendered. Unlike the display or the opacity properties, the visibility property does not appy directly to container elements, and therefore does not apply directly to the ‘use’ element. Because visibility is normally inherited, hiding the use element will often hide the child content, but not necessarily. If any graphics elements in the shadow tree have 'visibility:visible' specified, then that element will be visible even if the ‘use’ element specifies 'visibility:hidden'.

In the following example, key style rules are as follows:

.dark {
  visibility: hidden;
}
.eyes {
  visibility: visible;
}
svg:hover .dark, svg:focus .dark {
  visibility: visible;
}

The "dark" class is set on the group containing the ‘use’ elements, so all parts of the re-used graphics inherit the hidden visibility setting, except for the subtrees with class "eyes", where it is reset to visible. Upon hovering or focusing the graphic, the hiding effect is removed.

Example Use-visibility-hidden, default styles

Example Use-visibility-hidden, interactive styles

View this example as SVG

The example also demonstrates inheritance of other style properties (fill and stroke) specified on the ‘use’ elements, and how these are also not used if any elements within the symbol specify explicit values (e.g., the pink noses and ears and the white tails).

5.6.4. Multimedia in use-element shadow trees

When a use-element shadow tree includes ‘audio’ or ‘video’ elements, the following behaviors must be enforced:

• The user agent must synchronize playback of the media with the corresponding element (if it is rendered as part of the same document) and any other instances of the same corresponding element in the document.
• The instance must not play any audio if the ‘use’ element is not rendered.
• If user-agent provided playback controls are included for the corresponding element (or would be included, if the element was rendered), then they must be included for all instances in the document, and a user action on any instance must control playback on them all.

5.6.5. Animations in use-element shadow trees

The Web Animations API [web-animations-1] and the SVG Animations specification [svg-animation] define non-CSS ways to animate attributes and styles on targetted elements without directly manipulating DOM properties (see the Animation appendix for details). User agents that implement those features must ensure that all animations that apply to an element in a referenced document subtree also apply to instances of that element in a use-element shadow tree, as described in this section.

All animations within a use-element shadow tree operate in the same document timeline as for the corresponding use element, regardless of whether the referenced element is from the same or an external document.

For animation effects applied using a Web Animations API method [web-animations-1], if the target of the animation is a corresponding element to an element instance in a shadow tree, the user agent must construct a ShadowAnimation whose source is that Animation object and whose target is the element instance. If there are multiple instances of the element in different trees, then there will be multiple shadow animations, one for each.

The user agent must create such a ShadowAnimation for all Web Animations API animations in effect (including pending and frozen animations) at the time the shadow tree is generated, and for any new animations applied while the shadow tree exists. The user agent must not create ShadowAnimation objects for CSS animations or animation elements (as these are duplicated separately).

As part of the interface definition, a ShadowAnimation is read-only, and must reflect any changes to its sourceAnimation.

Any attempts to directly apply new animations to a target that is a read-only element instance (or pseudo-element) within a use-element shadow tree must throw a NoModificationAllowedError.

For each animation element [svg-animation] that targets an element in the referenced document subtree, the user agent must ensure that an equivalent animation element is in effect in the use-element shadow tree. If the animation element itself is part of the referenced document subtree, then this happens as a matter of course through the creation of an element instance for the animation element. Otherwise, the user agent must generate an element instance for the animation element that has the same effect as if it was a node in the shadow tree. The effective document order for these generated animation elements must be the same as the document order for their corresponding elements.

Each animation element or instance must only affect a target element in the same node tree (shadow or light), regardless of whether the targetting is implicit (the parent element) or explicit (a URL cross-reference to an element ‘id’). In this way, the one-to-one relationship between animation elements and target elements is preserved.

The ‘id’ attribute is cloned, like any other attribute, from the corresponding element to the element instance; This does not conflict with the requirement for ‘id’ to be unique, because the clone and the original are in distinct node trees.

All animation elements, in the document or in the shadow trees, which are timed to begin or end in response to an event on another element identified by its ‘id’ attribute, must also begin or end when any instance of an element with that ‘id’ receives the same event. This is consistent with how event listeners on a referenced element also listen to events on instances of that element, as described in the section on Event handling in use-element shadow trees. This behavior does not apply to animation begin or end times defined only by an event and not by an ‘id’ (and therefore implicitly listening for the event on the target element); in that case, each animation element is only triggered by its own target.

At the time an instance of an animation element is generated within a shadow tree, if there is an active animation associated with the corresponding element (including a frozen animation), and the timing event that initiated that animation would also have initiated the instance if it existed, then the animation for the element instance must be initiated, with its begin time adjusted backwards in the document timeline to match the timing of the corresponding element.

<set href="#target" begin="mouseover" ... />
  <!-- only affects the element that is moused over -->

<set href="#target" begin="target.mouseover" ... />
  <!-- affects all instances of the element with the id 'target',
          in all light and shadow node trees,
          when any of them are moused over -->
  

5.6.6. Event handling in use-element shadow trees

Element in a use-element shadow tree can both listen for and be the target of DOM events. Event retargetting provides encapsulation, so that the details of the shadow DOM structure are masked when an event bubbles out of the shadow tree and into the light.

Event retargeting is new in SVG 2. It provides consistency with the Shadow DOM specification, with existing implementations, and with the expectations of authors who are only concerned with elements in the main DOM.

Any event listeners defined on an element in the referenced graphics must also listen for the same event, at the same capture phase, on each instance of that element in a use-element shadow tree. This includes event listeners assigned using event attributes (which would be duplicated as with any other DOM attribute) and also event listeners assigned using the addEventListener method. The user agent must ensure that the list of event listeners for each element instance is synchronized to match its corresponding element. An event listener cannot be directly assigned to a read-only element instance in a use-element shadow tree. Any attempt to add an event listener to such an element must throw a NoModificationAllowedError.

Events in the use-element shadow tree are dispatched and bubble according to the shadow tree event path and event retargeting algorithm [DOM].

Any container element or graphics element in an SVG document can have zero or more ‘desc’ and/or ‘title’ elements as children, whose content is text. ‘desc’ and ‘title’ elements are not visually rendered as part of the graphics. The display value for the ‘title’ and ‘desc’ elements must always be set to none by the user agent style sheet, and this declaration must have importance over any other CSS rule or presentation attribute.

Multiple sibling ‘desc’ or ‘title’ elements must have different languages, as defined using a ‘lang’ attribute (or xml:lang attribute) on the descriptive element or an ancestor. The user agent must select the element of each type whose language best matches language preferences set by the user. A descriptive element with an empty-string language tag (indicating no language, for example a text alternative consisting of emoji symbols) is a lowest-priority match for any user, ranked below all user-specified language preferences. If multiple equally valid matches exist, the first match should be used. If no match exists for either 'title' or 'desc', the first element of that type must be selected.

<svg>
  <use href="#star">
    <title>Favourite</title>
    <title lang="en-us">Favorite</title>
    <title lang="nl">Favoriet</title>
    <title lang="">★</title>
  </use>
</svg>
  

The use of more than one ‘title’ or ‘desc’ element to provide localised information is at risk, with no known implementations.

User agents must make the text content of selected 'title' and 'desc' elements available to platform accessibility APIs as part of the name and description computation for the parent element, as defined in the SVG Accessibility API Mappings [SVG-AAM] specification.

Inclusion of any 'title' or 'desc' elements as a direct child of a rendered element indicates that the rendered element is of semantic importance in the graphic. Authors should not, and SVG generators must not, include empty 'title' or 'desc' elements with no text content or whitespace-only text content, as this will result in a nameless object being presented to assistive technology users.

If an individual graphic element has no meaning on its own, alternative text should instead be provided for the nearest container element that describes a meaningful object. Authors should use grouping (‘g’) elements to structure their drawing elements into meaningful objects, and name those groups with ‘title’. Conversely, if a container object is used simply to apply styles or layout, and neither defines an object nor provides meaningful grouping structure, it does not need alternative text.

Descriptive text elements whose parent is not rendered may be used by authors or authoring tools as reference information; authors are warned that this data is not normally available to end users viewing the graphic through assistive technologies. Nonetheless, a non-rendered element may be referenced as part of the accessible name or description of a rendered element (as defined in SVG-AAM), and the recursive computation will use descriptive child elements of the referenced element.

Description and title elements may contain marked-up text from other namespaces, using standard XML mechanisms to indicate the namespace. However, authors should not rely on such markup to provide meaning to alternative text; only the plain text content is currently required to be exposed to assistive technologies.

The HTML parser treats all markup within ‘title’ and ‘desc’ the same way it treats markkup in an HTML fragment; most elements will be assigned to the HTML namespace.

User agents may use markup within ‘title’ to influence the visual presentation of titles (such as tooltips), but are not required to do so.

The ‘title’ child element represents a short text alternative for the element.

On a link, this could be the title or a description of the target resource; on an image or drawing object, it could be a short description of the graphic; on interactive content, it could be a label for, or instructions for, use of the element; and so forth.

Authors should not provide redundant information in a ‘title’ element if there is also a visible label for the drawing element (e.g., using a ‘text’ element). Instead, the visual label should be associated with the drawing element using an ‘aria-labelledby’ attribute.

Interactive user agents should make the plain text content of ‘title’ elements available in response to user interaction, in a manner consistent with platform conventions; existing user agents commonly render ‘title’ elements as a tooltip on hovering the parent element.

Authors should provide a ‘title’ child element to the root svg element within a stand-alone SVG document. Since users often consult documents out of context, authors should provide context-rich titles. Thus, instead of a title such as "Introduction", which doesn't provide much contextual background, authors should supply a title such as "Introduction to Medieval Bee-Keeping" instead. For reasons of accessibility, user agents should always make the content of the ‘title’ child element to the root svg element available to users. However, this is typically done through other means than the tooltips used for nested SVG and graphics elements, e.g., by displaying in a browser tab.

The ‘desc’ element represents more detailed textual information for the element such as a description. This is typically exposed to assistive technologies to provide more detailed information, such as a description of the visual appearance of a graphic or help to explain the functionality of a complex widget. It is not typically available to other users, so should not be used for essential instructions.

Authors may associate detailed information, including visible text, with part of the graphic using ‘aria-describedby’ attribute (on the described element or a parent container), with the value being an ID reference to one or more SVG or HTML elements containing the description. The ‘aria-describedby’ attribute takes precedence over the child ‘desc’ when providing a description. If an element has both visible description and a child ‘desc’ element providing supplementary information, authors should explicitly include the ‘id’ of the element itself in its own ‘aria-describedby’ list, in order to concatenate the two descriptions together.

5.12.6. The ‘data-*’ attributes

All SVG elements support custom data attributes, which are those in no namespace whose names begin with the string "data-". See the requirements for custom data attributes in the HTML specification.

5.14.1. Extensions to the Document interface

The DOM Core specification defines a Document interface, which this specification extends.

In the case where an SVG document is embedded by reference, such as when an HTML document has an ‘object’ element whose ‘data’ attribute references an SVG document (i.e., a document whose MIME type is "image/svg+xml" and whose root element is thus an ‘svg’ element), there will exist two distinct DOM hierarchies. The first DOM hierarchy will be for the referencing document (e.g., an XHTML document). The second DOM hierarchy will be for the referenced SVG document.

partial interface Document {
  readonly attribute SVGSVGElement? rootElement;
};

The rootElement IDL attribute represents the root ‘svg’ element. On getting rootElement, the root element of the document is returned, if it is an ‘svg’ element, or null otherwise.

This attribute is deprecated, and may be removed in a future SVG specification. Authors are encouraged to use the documentElement attribute on Document instead.

SVG implementations that implement HTML must support the HTML extensions to the document interface. Other SVG implementations must support the following IDL fragment.

// must only be implemented in certain implementations
partial interface Document {
  readonly attribute DOMString title;
  readonly attribute DOMString referrer;
  readonly attribute DOMString domain;
  readonly attribute Element? activeElement;
};

The title, referrer, domain and activeElement IDL attributes must behave the same as the corresponding IDL attributes defined in HTML.

5.14.2. Interface SVGSVGElement

An SVGSVGElement object represents an ‘svg’ element in the DOM. The SVGSVGElement interface also contains miscellaneous utility methods, such as data type object factory methods.

An SVGSVGElement object maintains an internal DOMPoint object, called its current translate point object, which is the object returned from the currentTranslate IDL attribute.

[Exposed=Window]
interface SVGSVGElement : SVGGraphicsElement {

  [SameObject] readonly attribute SVGAnimatedLength x;
  [SameObject] readonly attribute SVGAnimatedLength y;
  [SameObject] readonly attribute SVGAnimatedLength width;
  [SameObject] readonly attribute SVGAnimatedLength height;

  attribute float currentScale;
  [SameObject] readonly attribute DOMPointReadOnly currentTranslate;

  NodeList getIntersectionList(DOMRectReadOnly rect, SVGElement? referenceElement);
  NodeList getEnclosureList(DOMRectReadOnly rect, SVGElement? referenceElement);
  boolean checkIntersection(SVGElement element, DOMRectReadOnly rect);
  boolean checkEnclosure(SVGElement element, DOMRectReadOnly rect);

  void deselectAll();

  SVGNumber createSVGNumber();
  SVGLength createSVGLength();
  SVGAngle createSVGAngle();
  DOMPoint createSVGPoint();
  DOMMatrix createSVGMatrix();
  DOMRect createSVGRect();
  SVGTransform createSVGTransform();
  SVGTransform createSVGTransformFromMatrix(DOMMatrixReadOnly matrix);

  Element getElementById(DOMString elementId);

  // Deprecated methods that have no effect when called,
  // but which are kept for compatibility reasons.
  unsigned long suspendRedraw(unsigned long maxWaitMilliseconds);
  void unsuspendRedraw(unsigned long suspendHandleID);
  void unsuspendRedrawAll();
  void forceRedraw();
};

SVGSVGElement includes SVGFitToViewBox;
SVGSVGElement includes SVGZoomAndPan;
SVGSVGElement includes WindowEventHandlers;

The x, y, width and height IDL attributes reflect the computed values of the x, y, width and height properties and their corresponding presentation attributes, respectively.

The currentScale and currentTranslate IDL attributes represent the transform applied to the document in response to user magnification and panning operations, as described under Magnification and panning.

The document's magnification and panning transform is a 2x3 matrix of the form [currentScale 0 0 currentScale currentTranslate.x currentTranslate.y]. The value of the ‘transform’ property does not affect currentScale or currentTranslate.

On getting currentScale, the following steps are run:

1. If the current ‘svg’ element is not the outermost svg element, then return 1.
2. Let [a b c d e f] be the 2x3 matrix that represents the document's magnification and panning transform.
3. Return a.

On setting currentScale, the following steps are run:

1. If the current ‘svg’ element is not the outermost svg element, then return.
2. Let scale be the value being assigned to currentScale.
3. Let [a b c d e f] be the 2x3 matrix that represents the document's magnification and panning transform.
4. Set the document's magnification and panning transform to [scale 0 0 scale e f].

On getting currentTranslate, the SVGSVGElement object's current translate point object is returned. This object represents the current translation for the ‘svg’ element. A current translate point object must be read only when its ‘svg’ element is not the outermost svg element, and writable otherwise.

See the rules for assigning to a DOMPoint for how modifying the current translate point object affects the document's magnification and panning transform.

Whenever the document's magnification and panning transform changes in response to user interaction or whenever the outermost svg element changes, the following steps are run:

1. Let [a b c d e f] be the 2x3 matrix that represents the document's magnification and panning transform.
2. Let element be the outermost svg element.
3. Update the x and y components of element's current translate point object to e and f, respectively.

Running these steps when the outermost svg element changes will ensure that if the document element is replaced with a different ‘svg’ element, that its currentTranslate will be immediately updated to reflect the translation component of the document's magnification and panning transform.

Whenever an ‘svg’ element is no longer outermost svg element, the x and y components of its current translate point object must be set to 0.

Note that the value of the ‘zoomAndPan’ attribute on the outermost svg element only controls whether the document's magnification and panning transform can be updated through user interaction. Regardless of the value of that attribute, the current scale and translation can be changed by modifying currentScale and currentTranslate.

The suspendRedraw, unsuspendRedraw, unsuspendRedrawAll and forceRedraw methods are all deprecated and defined to have no effect. When the suspendRedraw method is called, it must return 1.

The getIntersectionList, getEnclosureList, checkIntersection and checkEnclosure methods are used to perform geometry operations on graphics elements to find those whose (or check whether their) graphical content lies partially or completely within a given rectangle.

To find the intersecting or enclosed descendants of a given element element with a given rectangle rectangle using ancestor as the element in whose coordinate space rectangle is to be interpreted, the following steps are run:

1. Let result be an initially empty list.
2. If element is not displayed, due to having a display value of none or being in a subtree that has failing conditional processing attributes or a failing branch of a ‘switch’, then return result.
3. For each child element child of element, in document order:
   1. If child is an ‘svg’ or ‘g’ element, then:
      1. Let descendants be the result of finding the intersecting (or enclosed) descendants of child with rectangle in ancestor's coordinate space.
      2. Append to result all the elements of descendants.
   2. Otherwise, if child is a ‘use’ element, then:
      1. Let root be the root of the child's shadow tree.
      2. Let descendants be the result of finding the intersecting (or enclosed) descendants of root with rectangle in ancestor's coordinate space.
      3. If descendants is not empty, then append child to result.

         This means that although we look at the elements in the use-element shadow tree, we don't place the element instances or their corresponding element in the result list; only the ‘use’ element itself is returned.

   3. Otherwise, if child is a graphics element, then:
      1. Let region be the shape in child's coordinate system that is sensitive to hit detection, taking into account the rules for interpreting child's pointer-events value.
      2. Transform region into ancestor's coordinate system.
      3. If we are finding intersecting descendants and region lies partially or fully within rectangle, then append child to result.
      4. Otherwise, we are finding enclosed descendants. If region lies fully within rectangle, then append child to result.
4. Return result.

To find the non-container graphics elements within a given element element, the following steps are run:

1. Let result be an initially empty list.
2. If element is an ‘svg’ or ‘g’ element, then for each child element child of element, in document order:
   1. Let descendants be the result of finding the non-container graphics elements within child.
   2. Append to result all the elements of descendants.
3. Otherwise, if element is a graphics element then append element to result.
4. Return result.

When getIntersectionList(rect, referenceElement) or getEnclosureList(rect, referenceElement) is called, the following steps are run:

1. Let descendants be a list, depending on what method we are in:

   getIntersectionList

   descendants is the result of finding the intersecting descendants of the current ‘svg’ element with rectangle rect in the current ‘svg’ element's coordinate system.

   getEnclosureList

   descendants is the result of finding the enclosed descendants of the current ‘svg’ element with rectangle rect in the current ‘svg’ element's coordinate system.

2. If referenceElement is not null, then remove from descendants any element that does not have referenceElement as an ancestor.
3. Return a static NodeList that contains all of the elements in descendants. ([DOM], section 5.2.7)

When checkIntersection(element, rect) or checkEnclosure(element, rect) is called, the following steps are run:

1. Let descendants be a list, depending on what method we are in:

   getIntersectionList

   descendants is the result of finding the intersecting descendants of the current ‘svg’ element with rectangle rect in the current ‘svg’ element's coordinate system.

   getEnclosureList

   descendants is the result of finding the enclosed descendants of the current ‘svg’ element with rectangle rect in the current ‘svg’ element's coordinate system.

2. Let elements be the result of finding the non-container graphics elements within element.
3. If elements is empty, then return false.
4. If any element in elements is not also in descendants, then return false.
5. Return true.

The createSVGNumber, createSVGLength, createSVGAngle, createSVGPoint, createSVGMatrix, createSVGRect and createSVGTransform methods are all factory functions used to create a new datatype object of a particular type. When one of these methods is called, a new object is returned according to the following table:

The createSVGPoint, createSVGMatrix and createSVGRect methods are all deprecated and kept only for compatibility with legacy content. Authors are encouraged to use the DOMPoint, DOMMatrix and DOMRect constructors instead.

The createSVGTransformFromMatrix method is used to create a new SVGTransform object from a matrix object. Its behavior is the same as the createSVGTransformFromMatrix method on SVGTransformList.

5.14.3. Interface SVGGElement

An SVGGElement object represents a ‘g’ element in the DOM.

[Exposed=Window]
interface SVGGElement : SVGGraphicsElement {
};

5.14.4. Interface SVGUnknownElement

An SVGUnknownElement object represents an unknown element in the SVG namespace.

[Exposed=Window]
interface SVGUnknownElement : SVGGraphicsElement {
};

5.14.5. Interface SVGDefsElement

An SVGDefsElement object represents a ‘defs’ element in the DOM.

[Exposed=Window]
interface SVGDefsElement : SVGGraphicsElement {
};

5.14.6. Interface SVGDescElement

An SVGDescElement object represents a ‘desc’ element in the DOM.

[Exposed=Window]
interface SVGDescElement : SVGElement {
};

5.14.7. Interface SVGMetadataElement

An SVGMetadataElement object represents a ‘metadata’ element in the DOM.

[Exposed=Window]
interface SVGMetadataElement : SVGElement {
};

5.14.8. Interface SVGTitleElement

An SVGTitleElement object represents a ‘title’ element in the DOM.

[Exposed=Window]
interface SVGTitleElement : SVGElement {
};

5.14.9. Interface SVGSymbolElement

An SVGSymbolElement object represents a ‘symbol’ element in the DOM.

[Exposed=Window]
interface SVGSymbolElement : SVGGraphicsElement {
};

SVGSymbolElement includes SVGFitToViewBox;

New in SVG 2. The SVGSymbolElement interface now inherits from SVGGraphicsElement, so that the instantiated symbol in the shadow DOM can be queried as a graphics element.

5.14.10. Interface SVGUseElement

An SVGUseElement object represents a ‘use’ element in the DOM.

[Exposed=Window]
interface SVGUseElement : SVGGraphicsElement {
  [SameObject] readonly attribute SVGAnimatedLength x;
  [SameObject] readonly attribute SVGAnimatedLength y;
  [SameObject] readonly attribute SVGAnimatedLength width;
  [SameObject] readonly attribute SVGAnimatedLength height;
  [SameObject] readonly attribute SVGElement? instanceRoot;
  [SameObject] readonly attribute SVGElement? animatedInstanceRoot;
};

SVGUseElement includes SVGURIReference;

The x, y, width and height IDL attributes reflect the computed values of the x, y, width and height properties and their corresponding presentation attributes, respectively.

The instanceRoot and animatedInstanceRoot IDL attributes both point to the instance root, the SVGElementInstance that is a direct child of this element's shadow root (u.instanceRoot is equivalent to getting u.shadowRoot.firstChild). If this element does not have a shadow tree (for example, because its URI is invalid or because it has been disabled by conditional processing), then getting these attributes returns null.

5.14.11. Interface SVGUseElementShadowRoot

The root object of each use-element shadow tree implements the SVGUseElementShadowRoot interface. This interface does not currently define any extensions to the properties and methods defined for the ShadowRoot interface and DocumentOrShadowRoot mixin. However, the tree rooted at this node is entirely read-only from the perspective of author scripts.

[Exposed=Window]
interface SVGUseElementShadowRoot : ShadowRoot {
};

5.14.12. Mixin SVGElementInstance

The SVGElementInstance interface defines extensions to the SVGElement interface, which are only used for elements in a use-element shadow tree.

In previous versions of SVG, SVG element instances were defined as non-element objects that were valid event targets but not full DOM nodes. This specification re-defines the use-element shadow tree to be consistent with the Shadow DOM specification, which means that instances are actual SVGElement objects. This interface adds the missing functionality for backwards compatibility. However, authors should be aware that compatibility is not perfect, and design their scripts accordingly. Also note that these properties will not be available on HTML-namespaced element objects in the shadow tree.

interface mixin SVGElementInstance {
  [SameObject] readonly attribute SVGElement? correspondingElement;
  [SameObject] readonly attribute SVGUseElement? correspondingUseElement;
};

The correspondingElement IDL attribute points to the corresponding element if this element is an element instance in a use-element shadow tree, or is null otherwise.

When the referenced element is in an external file, the presence of this pointer implies that the entire DOM of the external file must be maintained in memory. However, as currently specified, the external DOM is read-only. It therefore offers limited functionality and a potentially large performance impact. Pending feedback from implementers, authors should consider the use of correspondingElement with external file references to be at-risk.

The correspondingUseElement IDL attribute points to the corresponding use element if this element is an element instance in a use-element shadow tree, or is null otherwise.

5.14.13. Interface ShadowAnimation

The ShadowAnimation inteface defines a read-only Animation object, which mirrors all changes to the sourceAnimation object from which it was constructed. They are used to mirror author-initiated animation objects in the use-element shadow tree.

[Constructor(Animation source, Animatable newTarget), Exposed=Window]
interface ShadowAnimation : Animation {
  [SameObject] readonly attribute Animation sourceAnimation;
};

The sourceAnimation IDL property points to the Animation object passed in the constructor.

The constructor generates a new ShadowAnimation object, which reflects all properties on the sourceAnimation, except that its effect is created by constructing a new KeyframeEffectReadOnly using the keyframe effect of the sourceAnimation as its source, and then modifying its target to match the newTarget parameter.

A ShadowAnimation is read-only. Any attempt to set any of the inherited IDL properties, or call any of the Animation methods that change its state, must throw a NoModificationAllowedError. However, the user agent must ensure that any changes to the properties or state of the sourceAnimation are reflected in changes to the ShadowAnimation.

5.14.14. Interface SVGSwitchElement

An SVGSwitchElement object represents a ‘switch’ element in the DOM.

[Exposed=Window]
interface SVGSwitchElement : SVGGraphicsElement {
};

5.14.15. Mixin GetSVGDocument

This interface provides access to an SVG document embedded by reference in another DOM-based language. The expectation is that the interface is implemented on DOM objects that allow such SVG document references.

This interface is deprecated and may be dropped from future versions of the SVG specification. To access the SVG document inside an ‘iframe’ or ‘object’ element, authors are suggested to use the contentDocument attribute on the HTMLIFrameElement or HTMLObjectElement interface, respectively.

The HTMLIFrameElement, HTMLEmbedElement and HTMLObjectElement interfaces all define their own getSVGDocument method, which provides access to the SVG document in the same way that the GetSVGDocument does. Those three interfaces therefore do not need to implement GetSVGDocument. Still, authors are strongly recommended to use contentDocument instead.

interface mixin GetSVGDocument {
  Document getSVGDocument();
};

The getSVGDocument method is used to return a referenced SVG document. When getSVGDocument() is called, it must return the Document object referenced by the embedding element that implements the GetSVGDocument interface; if there is no document, null is returned.

Note that this does no check to see whether the referenced document is indeed an SVG document. Instead, any document is returned.