CSS Scrollbars Styling Module Level 1

1. Introduction

This section is non-normative.

This CSS module introduces properties to influence the visual styling of scrollbars, including their color (scrollbar-color) and thickness (scrollbar-width).

1.1. Scope

The CSS Scrollbars Module is specifically for styling scrollbar controls themselves, e.g. their color & width in Level 1, and not their layout nor whether any content is scrollable. All layout impacts and content scrollability are specified in the CSS Overflow Module.

Based on documented use-cases, there are three main use-cases around scrollbars this module intends to resolve:

1. Coloring scrollbars to fit better into the UI of a web application.
2. Using a thinner scrollbar when the scrolling area is small.
3. Hiding UA-provided scrollbars, to allow the provision of custom interfaces for scrolling without affecting other aspects of scrollability.

1.1.1. Out Of Scope

The internal structure, layout, and configuration of scrollbars, as well as precise control over their coloring, is out of scope. This is because different platforms have different scrollbar structures and styling conventions, and operating systems continuously evolve their scrollbar designs to provide better user experience. Pseudo-elements for selecting specific parts of a scrollbar, for example, were considered and rejected. While this level of fine control would be tempting for authors, the arrangement of the various parts—​or whether they’re even all present—​cannot be depended on. Providing too much control would allow authors to get perfect results on some platforms, but at the expense of broken results on others.

Note: Exposing the scrollbar-related ::-webkit- prefixed pseudo-elements to the Web is considered a mistake by both the CSS Working Group and Webkit. Nevertheless, see § 2.1 Interaction with non-standard features and § 3.1 Interaction with non-standard features.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Scrollbar Colors: the scrollbar-color property

This property allows the author to set colors of an element’s scrollbars.

UAs must apply the scrollbar-color value set on the root element to the viewport.

Note: Unlike overflow (and overflow-*) properties, scrollbar-color value set on the HTML body element are not propagated to the viewport.

auto

The user agent determines the colors of the scrollbar. It should follow platform conventions, but may adjust the colors in accordance with color-scheme or other contextual information to better suit the page.

Tests

These tests check that that if the scrollbar adjusts based on the color scheme, then dynamic updates to the color scheme do cause an update of the scrollbar's colors.

• scrollbar-color-scheme-dynamic-1.html (live test) (source)
• scrollbar-color-scheme-dynamic-2.html (live test) (source)
• scrollbar-color-scheme-dynamic-3.html (live test) (source)
• scrollbar-color-scheme-dynamic-4.html (live test) (source)

---

<color>

The user agent should use the specified colors to adjust the rendering of the element’s scollbars, if any, applying the first color to the thumb of the scrollbar, and the second color to the track of the scrollbar.

The track refers to the background of the scrollbar, which is generally fixed regardless of the scrolling position.

The thumb refers to the moving part of the scrollbar, which usually floats on top of the track.

Some scrollbars also include additional controls, refered to as buttons, which usually sit at the top or bottom of the scrollbar on top of the track. This specification does not define whether and how buttons—​if present—​are colored.

If this property computes to a value other than auto, implementations may render a simpler scrollbar than the default platform UI rendering, and color it accordingly.

Note: Sometimes the UA is unable to customize the colors of native scrollbars, perhaps due to how they’re structured, or to a lack of control given by the native toolkit. The provision above allows the UA to replace them with differently-constructed scrollbars, which it does know how to color.

Note: User Agents are expected to use the color in the rendering of the scrollbar, but that does not imply they need to paint the entire thumb or track with a single solid featureless color. The specified color can be used as a tint for part of the component, lighter or darker shades of the color can be used for highlights, shadows, texture…

scrollbar-color: #8eb0dc #d2d4d6;

Implementations may ignore any of the colors if the corresponding part do not exist on the underlying platform.

When using scrollbar-color property with specific color values, authors should ensure the specified colors have enough contrast between them. For the auto value, UAs should ensure the colors they use have enough contrast. See WCAG 2.1 SC 1.4.11 Non-text Contrast [WCAG21]. UAs may ignore these contrast requirements based on explicit user preferences (for example, when users choose a configuration option/setting that always ensures a particular scrollbar color / use of system default scrollbars).

Note: Implementations can take advantage of the fact that they have full discretion in how to apply the specified colors in order to visually respond to user interactions with the scrollbar (e.g. hovering or activating): they could alter which scrollbar parts the specified colors apply to, use darker or lighter shades of the specified colors, more or less saturated variants…

If the user agent displays the scrollbar without the track, they must ensure reasonable contrast between the thumb and what is behind it. However, if the author set the color of the track to be fully transparent, the user agent may assume that the author has provided for sufficient contrast even with an invisible track.

Note: This is typically relevant in the case of overlay scrollbars, either on platforms where the track is never displayed, or on platforms where it is only displayed when the scrollbar is being interacted with but not when scrolling with the mouse wheel or the track-pad. This does not apply in cases where the entire scrollbar is hidden as is often the case with overlay scrollbars while no scroll operation is ongoing and when the scrollbar is not being interacted with either.

2.1. Interaction with non-standard features

On any element or pseudo-element where the computed value of scrollbar-color is anything other than the initial auto value, user agents must ignore any alternative non-standard means for authors to influence the rendering of scrollbars, such as the ::webkit-scrollbar family of pseudo-elements.

3. Scrollbar Thickness: the scrollbar-width property

This property allows the author to specify the desired thickness of an element’s scrollbars.

auto

Implementations must use the default scrollbar width.

Note: On most systems, this corresponds to the traditional somewhat wide scrollbar. However, through OS or UA settings, users can have the ability to change what this default corresponds to, possibly making the default scrollbar wider or narrower than is typical.

thin

Implementations should use thinner scrollbars than auto. This may mean a thin variant of scrollbar provided by the platform, or a custom scrollbar thinner than the default platform scrollbar. The scrollbar must nonetheless remain wide enough to be usable. (Implementers may wish to consult WCAG 2.1 SC 2.5.5 Target Size. [WCAG21])

Tests

• scrollbar-width-keywords.html (live test) (source)
• scrollbar-width-001.html (live test) (source)
• scrollbar-width-002.html (live test) (source)
• scrollbar-width-003.html (live test) (source)
• scrollbar-width-004.html (live test) (source)

Note: User agents can use various strategies to ensure the usability of narrow scrollbars. For instance, in the case of overlay scrollbars, they can dynamically enlarge the scrollbar in response to a user attempting to interact with it. User agents on devices with touch screens can also adjust how they interpret finger taps to facilitate interacting with visually small touch targets.

User agents may disregard this value and treat it as auto, for instance when the user has indicated discomfort for thin scrollbars through some UA or OS setting. (User agents are encouraged to provide such a setting.)

Note: Some platforms only have a tiny scrollbar by default which cannot be reasonably made thinner. In such cases, this value will behave as auto.

none

Implementations must not display any scrollbar, however the element’s scrollability by other means is not affected.

Tests

• textarea-scrollbar-width-none.html (live test) (source)
• scrollbar-width-keywords.html (live test) (source)
• scrollbar-width-001.html (live test) (source)
• scrollbar-width-002.html (live test) (source)
• scrollbar-width-003.html (live test) (source)
• scrollbar-width-004.html (live test) (source)
• scrollbar-width-007.html (live test) (source)

Using this value can prevent mouse-only users from being able to scroll. Authors should ensure that mouse-only users can still reach hidden content, even if they have no scrollwheel.

Authors that use none should provide an alternative/equivalent visual hint that scrolling is possible and there is more content.

Note: For situations where an element is to be scrolled only by programmatic means, and not by direct user manipulation, overflow: hidden is appropriate, not scrollbar-width: none.

* { scrollbar-width: auto !important; }

UAs must apply the scrollbar-width value set on the root element to the viewport.

Note: Unlike the overflow property (and its longhands), a scrollbar-width value set on the HTML body element is not propagated to the viewport.

Note: This specification does not define the exact position or shape of the scrollbar, or any animation thereof, such as fading or sliding in/out of view.

3.1. Interaction with non-standard features

On any element or pseudo-element where the computed value of scrollbar-width is anything other than the initial auto value, user agents must ignore any alternative non-standard means for authors to influence the rendering of scrollbars, such as the ::webkit-scrollbar family of pseudo-elements.

Appendix A. Acknowledgments

This appendix is non-normative.

The CSS Working Group and the Editors of this specification would like to express our gratitude to the following people, as this specification would not have been possible without their feedback and contributions: Chris Lilley, Emilio Cobos Álvarez, Eric Shepherd, Eric Willigers, fantasai, Greg Whitworth, jonjohnjohnson, L. David Baron, Luke Warlow, Oriol Brufau, Patrick H. Lauke, Rene Haas, Rick Byers, Simon Fraser, Tab Atkins Jr., Xidorn Quan.

In particular, thanks for the use-cases, prototyping, implementation, and feedback from Tab Atkins and Xidorn Quan. Thanks for accessibility review and contributions (#3315) from Patrick H. Lauke.

Appendix B. Changes

This appendix is non-normative.

Changes since the 9 December 2021 Candidate Recommendation

• Switched the Animation type of scrollbar-width to discrete, reflecting the fact that this property only takes keyword values.

Changes since the 2021-12-02 Working Draft

• Boilerplate changes for CR

Changes from the 2021-08-05 Working Draft

• Switched "should" to a "must" with regards to accessibility of narrow scrollbars. (see Issue 6675)

Changes from the 2018-09-25 First Public Working Draft

• #6538: removed light and dark values of scrollbar-color in favor of allowing the UA to tune auto in accordance with color-scheme or other contextual information.
• #3237: scrollbar-color computed value changed to: specified keyword or two computed colors
• #4693: Clarified scope: styling scrollbar controls themselves, no layout or scrollability.
• #3315: More and updated accessibility considerations for scrollbar-color and scrollbar-width.

Appendix C. Security and Privacy

This appendix is non-normative.

Considerations for Security

No specific concerns regarding security have been identified for this specification.

Considerations for Privacy

No specific concerns regarding privacy have been identified for this specification.

Self-review questionnaire

Per the Self-Review Questionnaire: Security and Privacy: Questions to Consider

1. Does this specification deal with personally-identifiable information?

   No.

2. Does this specification deal with high-value data?

   No.

3. Does this specification introduce new state for an origin that persists across browsing sessions?

   No.

4. Does this specification expose persistent, cross-origin state to the web?

   No.

5. Does this specification expose any other data to an origin that it doesn’t currently have access to?

   No.

6. Does this specification enable new script execution/loading mechanisms?

   No.

7. Does this specification allow an origin access to a user’s location?

   No.

8. Does this specification allow an origin access to sensors on a user’s device?

   No.

9. Does this specification allow an origin access to aspects of a user’s local computing environment?

   No.

10. Does this specification allow an origin access to other devices?

    No.

11. Does this specification allow an origin some measure of control over a user agent’s native UI?

    Yes. The scrollbar-* properties enable the page to change the color and width of the scrollbar of the user agent’s native UI, e.g. scrollbars on the page’s window, on framed content embedded in the page, or on overflowing elements with scrollbars in the page.

12. Does this specification expose temporary identifiers to the web?

    No.

13. Does this specification distinguish between behavior in first-party and third-party contexts?

    No.

14. How should this specification work in the context of a user agent’s "incognito" mode?

    No differently.

15. Does this specification persist data to a user’s local device?

    No.

16. Does this specification have a "Security Considerations" and "Privacy Considerations" section?

    Yes.

17. Does this specification allow downgrading default security characteristics?

    No.

Appendix D. Considerations for accessibility

This appendix is non-normative.