Grid Layout

2.1 Basic Capabilities of the Grid

As websites evolved from simple documents into complex, interactive applications, tools for document lay out, e.g. floats, were not necessarily well suited for application lay out. By using a combination of tables, JavaScript, or careful measurements on floated elements, authors discovered workarounds to achieve desired layouts. Layouts that adapted to the available space were often brittle and resulted in counter-intuitive behavior as space became constrained. As an alternative, authors of many web applications opted for a fixed layout that cannot take advantage of changes in the available rendering space on a screen.

The lay out capabilities of the Grid address these problems. The Grid provides a mechanism for authors to divide available space for lay out into columns and rows using a set of predictable sizing behaviors. Authors can then precisely position and size the building block elements of their application by referencing the Grid Lines between the columns and rows, or by defining and referencing a Grid Cell, which is a rectangular space covering an intersection of columns and rows. Figure 1 illustrates a basic layout which can be achieved with the Grid.

2.2 Adapting Layouts to Available Space

The Grid element can be used to intelligently reflow elements within a webpage. Figure 2 represents a game with five major areas in the layout: the game title, stats area, game board, score area, and control area. The author's intent is to divide the space for the game such that:

• The stats area always appears immediately under the game title.
• The game board appears to the right of the stats and title.
• The top of the game title and the game board should always align.
• The bottom of the game board and the stats area align when the game has reached its minimum height, but otherwise the game board will stretch to take advantage of all the screen real-estate available to it.
• The score area should align into the column created by the game and stats area, while the controls are centered under the board.

As an alternative to using script to control the absolute position, width, and height of all elements, the author can use the Grid element, as shown in Figure 3. The following example shows how an author might achieve all the sizing, placement, and alignment rules declaratively.

Note that there are multiple ways to specify the structure of the Grid and to position and size Grid Items, each optimized for different scenarios. This example illustrates one that an author may use to define the position and space for each Grid Item using the grid-rows and grid-columns properties of the Grid element, and the grid-row, grid-column, grid-row-span and grid-column-span properties on each Grid Item.

<style type="text/css">
    #grid     { display: grid; 
                /* Two columns: the first sized to content, the second receives the remaining space,   */
                /* but is never smaller than the minimum size of the board or the game controls, which */
                /* occupy this column. */
                grid-columns: auto minmax(min-content, 1fr);

                /* Three rows: the first and last sized to content, the middle row receives the remaining */
                /* space, but is never smaller than the minimum height of the board or stats areas.       */
                grid-rows: auto minmax(min-content, 1fr) auto
              }

    /* Each part of the game is positioned between grid lines by referencing the starting grid line and */
    /* then specifying, if more than one, the number of rows or columns spanned to determine the ending */
    /* grid line, which establishes bounds for the part. */
    #title    { grid-column: 1; grid-row: 1 }
    #score    { grid-column: 1; grid-row: 3 }
    #stats    { grid-column: 1; grid-row: 2; grid-row-align: start }
    #board    { grid-column: 2; grid-row: 1; grid-row-span: 2 }
    #controls { grid-column: 2; grid-row: 2; grid-column-align: center }
</style>

<div id="grid">
    <div id="title">Game Title</div>
    <div id="score">Score</div>
    <div id="stats">Stats</div>
    <div id="board">Board</div>
    <div id="controls">Controls</div>
</div>

2.3 Source Independence

Continuing the prior example, the author also wants the game to adapt to the space available on traditional computer monitors, handheld devices, or tablet computers. Also, the game should optimize the placement of the components when viewed either in landscape or portrait orientation (Figures 4 and 5). By combining the CSS markup for the Grid element with media queries, the author is able to use the same semantic markup, but rearranged independent of its source order, to achieve the desired layout in both orientations.

The following example leverages the Grid's ability to name the space which will be occupied by a Grid Item. This allows the author to avoid rewriting rules for Grid Items as the Grid's definition changes.

<style type="text/css">
    @media (orientation: portrait) {
        #grid { 
            display: grid;
            
            /* The rows, columns and cells of the grid are defined visually using the grid-template property.        */
            /* Each string is a row, and each letter a cell.  The max number of letters in any one string determines */
            /* the number of columns. */
            grid-template: "ta"
                           "sa"
                           "bb"
                           "cc";
            
            /* Columns and rows created with the template property can be assigned a sizing function with the */
            /* grid-columns and grid-rows properties. */
            grid-columns: auto minmax(min-content, 1fr); 
            grid-rows: auto auto minmax(min-content, 1fr) auto
        }
    }

    @media (orientation: landscape) {
        #grid { 
            display: grid;
            
            /* Again the template property defines cells of the same name, but this time positioned differently */
            /* to better suit a landscape orientation. */
            grid-template: "tb"
                           "ab"
                           "sc";
            
            grid-columns: auto minmax(min-content, 1fr); 
            grid-rows: auto minmax(min-content, 1fr) auto
        }
    }

    /* The grid-cell property places a grid item into named region (cell) of the grid. */
    #title    { grid-cell: "t" }
    #score    { grid-cell: "s" }
    #stats    { grid-cell: "a" }
    #board    { grid-cell: "b" }
    #controls { grid-cell: "c" }
</style>

<div id="grid">
    <div id="title">Game Title</div>
    <div id="score">Score</div>
    <div id="stats">Stats</div>
    <div id="board">Board</div>
    <div id="controls">Controls</div>
</div>

2.4 Grid Layering of Elements

In the example shown in Figure 6, the author is creating a custom slider control. The control has six parts. The lower and upper labels align to the left and right edges of the control. The track of the slider spans the area between the labels. The lower and upper fill parts touch beneath the thumb, and the thumb is a fixed width and height that can be moved along the track by updating the two fraction-sized columns.

Prior to the Grid element, the author would have likely used absolute positioning to control the top and left coordinates, along with the width and height of each HTML element that comprises the control. The z-index property would have been used to control element drawing order. By leveraging the Grid element, the author can instead limit script usage to handling mouse events on the thumb, which snaps to various positions along the track as the grid-columns property of the Grid element is updated. The grid-layer property provides capabilities which are analagous to z-index for Grid Items.

<style type="text/css">
    #grid { 
        display: grid;

        /* The grid-columns and rows properties also support naming grid lines which can then be used */
        /* to position grid items.  The line names are assigned on either side of a column or row     */
        /* sizing function where the line would logically exist. */
        grid-columns:      
            "start"        auto 
            "track-start"  0.5fr 
            "thumb-start"  auto 
            "fill-split"   auto 
            "thumb-end"    0.5fr 
            "track-end"    auto
            "end";
    }

    /* Grid-column and grid-row accept a starting and optional endling line.  Below the lines are referred to by name. */
    /* Beyond any semantic advantage, the names also allow the author to avoid renumbering the grid-row and column     */
    /* properties of the grid items.  This is similar to the concept demonstrated in the prior example with the        */
    /* grid-template property during orientation changes, but grid lines can also work with layered grid items that    */
    /* have overlapping cells of different shapes like the thumb and track parts in this example. */
    #lower-label { grid-column: "start" }
    #track       { grid-column: "track-start" "track-end"; grid-row-align: center }
    #upper-label { grid-column: "track-end"; }
    
    /* Fill parts are drawn above the track so set grid-layer to 5. */
    #lower-fill  { grid-column: "track-start" "fill-split"; grid-row-align: center; grid-layer: 5 }
    #upper-fill  { grid-column: "fill-split" "track-end"; grid-row-align: center; grid-layer: 5 }
    
    /* Thumb is the topmost part; assign it the highest grid-layer value. */
    #thumb       { grid-column: "thumb-start" "thumb-end"; grid-layer: 10 }
</style>

<div id="grid">
    <div id="lower-label">Lower Label</div>
    <div id="upper-label">Upper Label</div>
    <div id="track">Track</div>
    <div id="lower-fill">Lower Fill</div>
    <div id="upper-fill">Upper Fill</div>
    <div id="thumb">Thumb</div>
</div>

3.1 Grid Tracks

Grid Tracks are the columns and rows of the Grid defined with the grid-rows and grid-columns properties on the Grid element. Each Track is defined by declaring a sequential list of sizing functions, one for each Track. Tracks define the space between Grid Lines.

<style type="text/css">
    #grid { 
        display: grid; 
        grid-columns: 150px 1fr; /* two columns */
        grid-rows: 50px 1fr 50px /* three rows  */
    }
</style>

3.2 Grid Lines

Grid Lines are the horizontal or vertical dividing lines that exist on either side of a column or row. Grid Lines may be referred to by their Grid Line number, or they may be named by the author. Authors use one or more quoted strings to assign names to a Line before or after a Track definition wherever the desired Line would logically exist. A Grid Item then uses the Grid Lines to determine its position and available space within the Grid by referencing the Grid Lines using the properties grid-row and grid-column. Grid-row and grid-column accept a starting and optional ending Line.

The following two examples create three column Grid Lines and four row Grid Lines. The first example demonstrates how an author would refer to the Grid Lines using Grid Line numbers. The second example uses explicitly named Grid Lines.

<style type="text/css">
    #grid { 
        display: grid; 
        grid-columns: 150px 1fr;
        grid-rows: 50px 1fr 50px
    }

    #item1 { grid-column: 2; grid-row: 1 4 }
</style>

<style type="text/css">
    /* equivalent layout to the prior example, but using named lines */
    #grid { 
        display: grid; 
        grid-columns: 150px "item1-start" 1fr "item1-end";
        grid-rows: "item1-start" 50px 1fr 50px "item1-end"
    }

    #item1 { 
        grid-column: "item1-start" "item1-end"; 
        grid-row: "item1-start" "item1-end" 
    }
</style>

3.3 Grid Cells

Grid Cells are the logical space used to lay out one or more Grid Items. Grid Cells may be defined explicitly using the grid-template property or the grid-cell pseudo-element. The grid-cell pseudo-element uses a functional syntax which accepts a string name for the Cell. The space for the explicitly defined Cell is then defined by referencing Grid Lines. When a Grid Cell is explicitly defined, multiple Grid Items may be stacked or layered into the same Grid Cell. When a Grid Item refers to Grid Lines directly, an anonymous Grid Cell is created which contains only the single Grid Item.

<style type="text/css">
    #grid  { 
        display: grid; 
        grid-columns: 150px "cell-start" 1fr "cell-end";
        grid-rows: "cell-start" 50px 1fr 50px "cell-end"
    }

    #grid::grid-cell("cell") { 
        grid-column: "cell-start" "cell-end"; 
        grid-row: "cell-start" "cell-end"
    }

    #item2 { grid-cell: "cell" }
    #item3 { grid-cell: "cell" }
</style>

<style type="text/css">
    /* equivalent layout compared to the prior, but using the template syntax */
    #grid  { 
        display: grid; 
        grid-template: "ad"
                       "bd"
                       "cd";
        grid-columns: 150px 1fr;
        grid-rows: 50px 1fr 50px
    }

    #item2 { grid-cell: "b" }
    #item3 { grid-cell: "b" }
</style>

6.1 Grid Rows, Columns and Implicit Grid Lines

Grid elements use Grid Lines to divide their space.  There are two sets of Grid Lines: one set defined by the columns that run in the direction of block progression, and another orthogonal set defined by rows.  Block progression is a writing-mode term that defines a logical direction.  In English it means vertical.

A Grid Track is a generic term for a column or row which separates two Grid Lines. Each Grid Track is assigned a sizing function, which controls how wide or tall the column or row may grow, and thus how far apart two Grid Lines are.  The sizing function specified can be a length, a percentage of the Grid element's size, derived from the contents occupying the column or row, or a proportion of the space which remains in the Grid element.  In the last case, remaining space refers to the width or height of the Grid element after accounting for space already consumed by columns and rows sized with a length, percentage or content.  The size can also be specified as a range using a minmax function, which can combine any of the previously mentioned mechanisms to define a min and max size for the column or row.  

In the following example there are two columns and three rows.  The first column is 150px wide beginning from the starting edge of the Grid element's content box.  The second column uses fractional sizing, which is a function of the remaining space in the Grid.  Its size will vary as the width of the Grid element changes.  If the used width of the Grid element is 200px, then the second column 50px wide.  If the used width of the Grid element is 100px, then the second column is 0px and any content positioned in the column will be overflowing the Grid element.  Sizing occurs in a similar fashion for the rows.

<style type="text/css">
    #grid { 
        display: grid; 
        grid-columns: 150px 1fr;
        grid-rows: 50px 1fr 50px
    }
</style>

6.2 Named Grid Lines

A Grid Line exists on either side of a column or row.  The Grid Line may be named using one or more quoted strings which are positioned in the grid-rows or grid-columns definitions where the Grid Line would logically occur (in between the sizing functions that define the Grid's columns and rows).  Each name associated with a Grid Line must be unique for the set of columns or rows.  If the name is specified multiple times in the same column or row definition, it is associated with the first Grid Line to which the name was assigned. When a name is not specified, Grid Lines can be referred to in the order which they occur.  The first line is 1, the second 2 and so on.  The next example builds on the prior by assigning each line one or more names.

<style type="text/css">
    #grid {
        display: grid;
        grid-columns: "first" "nav" 150px "main" 1fr "last";
        grid-rows: "first" "header" 50px "main" 1fr "footer" 50px "last";
    }
</style>

6.3 Starting and Ending Grid Lines

In addition to any explicitly named Grid Lines defined by the author, a Grid element also has four implicitly defined Grid Lines: a start and end line for both columns and rows.  The start line is always positioned on the starting edge of the Grid element's content box.  The end line is positioned at the ending edge of the Grid element's content box, or at the same location as the last explicitly or implicitly defined Grid Line when it extends past the content box of the Grid element.  

In the following example, the 'end' column Grid Line is positioned on the ending edge of the Grid element's content box.  The 'end' row Grid Line is positioned past the ending edge in overflow at the same location as the Grid Line named "last."

 

<style type="text/css">
    #grid {
        display: grid;
        width: auto;
        height: 500px;
        grid-columns: 50px 1fr;
        grid-rows: "first" 250px 250px 250px "last";
    }

    #item {
        /* cover explicitly defined columns and rows */
        grid-column: 1 3;
        grid-row: "first" "last";

        /* equivalent (assuming no implicitly created tracks) */
        grid-column: start end;
        grid-row: start end;
    }
</style>

6.4 Repeating Columns and Rows

If there are large number of columns or rows that are the same or exhibit a recurring pattern, a repeat syntax can be applied to define the columns or rows in a more compact form.

The next two examples are equivalent.  There is a single row, and a pattern of repetitive column Grid Lines: a 250px column followed by a 10px gutter.  Note that when repeat syntax is used with Grid Line naming, that the names are assigned to the first occurrence of the pattern.

 

<style type="text/css">
    #grid {
        display: grid;
        grid-columns: 10px "content" 250px 10px 250px 10px 250px 10px 250px 10px;
        grid-rows: 1fr;
    }

    /* Equivalent definition. */
    #grid {
        display: grid;
        grid-columns: 10px ("content" 250px 10px)[4];
        grid-rows: 1fr;
    }
</style>

 

6.5 Grid-columns and Grid-rows Properties

The following grammar expresses the allowable values for the grid-rows and grid-columns properties.

<track-list>    => [ <track-group> ]+ | 'none'
<track-group>   => [ <string>* '(' <string>* [ <track-minmax> ]+ <string>* ')' [ '[' <positive-number> ']' ]? <string>*
                   | <string>* <track-minmax> <string>*
<track-minmax>  => 'minmax(' <track-breadth> ',' <track-breadth> ')' | 'auto' | <track-breadth>
<track-breadth> => <length> | <percentage> | <fraction> | 'min-content' | 'max-content'

Where:

• length is as defined by CSS3 Values. [CSS3VAL]
• percentage expresses a size for a Grid Track as a percentage of the Grid element's logical width (for columns) or logical height (for rows). When the width or height of the Grid element is undefined, the percentage is ignored and the Grid Track will be auto-sized.
• fraction is a non-negative floating-point number followed by 'fr'. Each fraction value takes a share of the remaining space proportional to its number. See fraction values for more details.
• max-content is a keyword which refers to the maximum of the max sizes of Grid Items occupying the Grid Track.
• min-content is a keyword which refers to the maximum of the min sizes of Grid Items occupying the Grid Track.
• minmax(min, max) defines a size range. The size used should be greater than or equal to min and less than or equal to max. If max < min, then max is ignored and minmax(min,max) is treated as minmax(min,min).
• fit-content is equivalent to minmax(min-content, max-content).
• auto is quivalent to fit-content.

Flex should also be allowed as a sizing function for a Grid Track. Update the grammar to accept the flex function when the Flexbox spec stabilizes.

Name:	grid-columns
Value:	see grammar above
Initial:	none
Applies to:	non-replaced elements with a computed value of 'grid' or 'inline-grid' for display.
Inherited:	no
Percentages:	n/a
Media:	visual, paged
Computed value:	see text

Name:	grid-rows
Value:	see grammar above
Initial:	none
Applies to:	non-replaced elements with a computed value of 'grid' or 'inline-grid' for display.
Inherited:	no
Percentages:	n/a
Media:	visual, paged
Computed value:	see text

The following example:

• Adds one Grid Line 100 pixels from the 'start' of the grid.
• Adds another Grid Line 1/2 of the remaining space away.
• Adds another Grid Line whose distance from the prior Grid Line will be based on the maximum content size of elements occupying the column.
• Adds another Grid Line whose distance from the prior Grid Line is based on the minimum content size of all elements occupying the column, or 1/2 the remaining space, whichever is greater.

div { grid-columns: 100px 1fr max-content minmax(min-content, 1fr) }

Additional examples of valid Grid Track definitions:

    /* examples of valid track definitions */
    grid-rows: 1fr minmax(min-content, 1fr);
    grid-rows: 10px (1fr auto minmax(30%, 1fr))[2];
    grid-rows: (10px);
    grid-rows: calc(4em - 5px)

6.5.1 Fraction Values: 'fr'

Fraction values are new units applicable to the grid-rows and grid-columns properties:

fr

Fraction of available space.

The distribution of fractional space occurs after all <length> or content-based row and column sizes have reached their maximum. The total size of the rows or columns is then subtracted from the available space and the remainder is divided proportionately among the fractional rows and columns.

Each column or row's proportional share can be computed as the column or row's <fraction> * <remaining space> / <sum of all fractions>. Note that fractions occurring within a minmax function are only counted in the sum if in the max position. Further, fractions that occur in the min position are treated as an absolute length of 0px.

When remaining space cannot be determined because the width or height of the Grid element is undefined, fraction-sized Grid Tracks are sized to their contents while retaining their respective proportions. In such cases the size of each fractional Grid Track can be computed by determining the max-content size of each fractional Grid Track and dividing that size by the respective fraction. The maximum value is of those results is treated as the 1fr value, which is then multiplied by each Grid Track's fraction to determine its final size.

6.5.2 Computed Values for Grid-rows and Grid-columns

The computed size of all Grid Tracks as returned for the grid-rows and grid-columns properties are normalized to used pixel values. All Grid Tracks are included in the computed value reported for grid-rows and grid-columns regardless of how the Grid Tracks were created, e.g. implicit tracks may be created by Grid Items referencing a Grid Line not explicitly defined by a grid-rows or grid-columns property. The computed value includes any used values for named lines; any duplicate names must be removed. User agents may use the repeat syntax to avoid expansive string length due to implicit Grid Track creation, large spanning values or high repeat counts specified by the author.

The algorithm used to apply repeat syntax to used Grid Track values and Grid Line names is User agent specific. See below for example inputs and outputs.

<style type="text/css">
    #grid {
        width: 500px;
        grid-columns: "a" auto "b" minmax(min-content, 1fr) "b" "c" "d" ("e" 40px)[2] (auto)[5];
    }
</style>
<div id="grid">
    <div style="grid-column:1; width:50px"></div>
    <div style="grid-column:9; width:50px"></div>
</div>
<script type="text/javascript">
    // Returns '"a" 50px "b" 320px "c" "d" ("e" 40px)[2] (0px)[4] 50px'.
    window.getComputedStyle(document.getElementById("grid"), null).getPropertyValue("grid-columns")
</script>

7.1 Anonymous Grid Cells

Grid Items are placed into Grid Cells. A Grid Cell defines the available space for a Grid Item.  The dimensions of the Grid Cell are specified by naming the starting and ending lines which define the columns and rows that the Grid Cell occupies.  The starting and ending lines may be referred to by a string name, if one was defined by the author, the start and end line keywords, or the Grid Line's number.

The following example positions the first Grid Item to cover all rows and columns of the Grid element using the start and end keywords.  The second Grid Item is positioned to cover the first row.  Note that when only the starting Grid Line of the grid-row or grid-column properties are specified for the Grid Cell, that the Grid Cell is assumed to extend to the subsequent Grid Line.

<style type="text/css">
    /* covers the Grid element's content box */
    #item1 {
        grid-row: start end;
        grid-column: start end;
    }
 
    /* covers the first row */
    #item2 {
        grid-row: start; /* extends to row Grid Line 2 */
        grid-column: start end;
    }
</style>

The next example defines rows for header and footer Grid Items sized to content, as well as a main region that receives all remaining space.   A single column which receives all horizontal space in the Grid's content box is also defined.

<style type="text/css">
    #grid {
        display: grid;
        grid-rows: "header" auto "main" 1fr "footer" auto;
        grid-columns: 1fr;
    }
 
    #header { grid-row: "header"; grid-column: start }
    #main   { grid-row: "main"; grid-column: start }
    #footer { grid-row: "footer"; grid-column: start }

    /* Equivalent to the above using grid line numbers instead of names. */
    #header { grid-row: 1; grid-column: 1 }
    #main   { grid-row: 2; grid-column: 1 }
    #footer { grid-row: 3; grid-column: 1 }
</style>

7.2 Explicitly Defined Grid Cells

The prior example created an anonymous Grid Cell that holds a single Grid Item. Grid Cells can be created explicitly using the grid-cell pseudo element function. The function accepts as its only parameter a string name to associate with the Grid Cell. Grid Cells are positioned and sized using the grid-row and grid-column properties. Once defined, multiple Grid Items can be placed into the Grid Cell by referring to it by name using the grid-cell property.

When multiple items are placed into a single Grid Cell, they can be stacked or layered. Layering places each Grid Item over the other in accordance with the grid-layer property (described in Drawing Order of Grid Items). Stacking places Grid Items one after the other in the direction of increasing column or row numbers depending on the value of the grid-cell-stacking property.

Grid-cell-stacking introduces behavior that is similar to Flexbox. Why not just allow an author to specify a display on explicitly defined Grid Cells to control how its contents lay out instead? Setting #grid::grid-cell("foo") { display:flexbox } would trigger creation of an anonymous Flexbox that performs lay out on every item placed into the Grid Cell as though they were Flex Items. display:block would cause items to lay out as though they were part of the normal flow. What type of display would allow behavior that is consistent with the default layering though? display: grid-item?

The following example explicitly defines a Grid Cell 'nav' and stacks all child links of the Grid element into the 'nav' Grid Cell.

<style type="text/css">
    #grid {
        grid-columns: auto 1fr;
        grid-rows: auto 1fr;
    }
    #grid::grid-cell("nav") {
        grid-column: start; /* first column */
        grid-row: start end; /* all rows */
        
        /* stack items placed into this cell one after the other in the 'rows' direction */
        grid-cell-stacking: rows;
    }
    #grid > a {
        display:block; /* make anchor tags valid Grid Items */
        grid-cell: "nav";
    }
</style>

7.3 Defining Grid Cells with a Template

Grid Cells can also be defined explicitly using one-letter names via the grid-template property. The grid-template property provides a visualization of the Grid element's structure while simultaneously defining the Grid Cells which can be used to layer or stack the child elements of the Grid.

In the following example, a grid-template property is used to create a page layout where cells are defined for header content 'h', navigational content 'n', footer content 'f', and main content 'm'. In English, a row is created for every separate string listed for the grid-template property, and a column is created for the maximum number of letters found in any one string. In orthogonal writing modes the roles of rows and columns are reversed. See Grid Writing Modes for more details.

<style type="text/css">
    #grid {
        display: grid;
        grid-template: "hh"
                       "nm"
                       "ff"
    }
    #grid > a {
	    display:block; /* make anchor tags valid Grid Items */
	    grid-cell: "n";
    }
</style>

<style type="text/css">
#item {
	/* the following two property definitions are equivalent */
	/* both place the item between the first and third line */
	/* which is covering the first and second row of the Grid */
	grid-row: 1 3;
	grid-row: 1; grid-row-span: 2;
}
</style>

7.5 Implicit Columns and Rows

Grid Line numbers referred to by grid-row or grid-column properties on a Grid Item or Grid Cell are not required to refer to a Grid Line that was defined using the grid-columns or grid-rows properties on the Grid element.  In cases where the specified position is outside those explicitly specified on the Grid element, implicit columns and rows fill gaps added as needed to create additional Grid Lines until the reference can be resolved. Note that quoted Grid Line names that don't resolve to an explicitly defined Grid Line are treated as thought the author specified the 'start' Grid Line and don't trigger implicit Grid Track creation.

Figure 10 illustrates the placement of Grid Items resulting from the markup shown in the following example. Note that Grid Item 'B' is positioned on Grid Line 5 which creates a 5th column to contain Grid Item 'B'.  Further, columns 3 and 4 are created as implicit auto-width columns without content resulting in a used column width of 0px.  

<style type="text/css">
    #grid { display: grid; grid-columns: 20px; grid-rows: 20px }
    #A { grid-column: 1; grid-row: 1; grid-column-align: start; grid-row-align: start }
    #B { grid-column: 5; grid-row: 1; grid-row-span: 2; }
    #C { grid-column: 1; grid-row: 2; grid-column-span: 2; }
</style>

<div id="grid">
    <div id="A">A</div>
    <div id="B">B</div>
    <div id="C">C</div>
</div>

7.6 Automatic Placement of Grid Items

This section describes early thinking around automatic placement of Grid Items. Multiple algorithms are possible for such a feature. One is proposed here.

Grid Items can be automatically placed into an unoccupied space of the Grid. The grid-flow property controls the direction in which the search for unoccupied space takes place, and whether rows or columns are added as needed to accomodate the content. Note that Grid Cells cannot be automatically placed.

A grid-flow value of 'rows' will search across columns and then rows for unoccupied space, and will create additional rows as needed to accomodate content. Similarly, a grid-flow value of 'columns' searches rows first and creates additional columns as needed until sufficient space is found.

Name:	grid-flow
Value:	'none' | 'rows' | 'columns'
Initial:	none
Applies to:	Grid elements
Inherited:	no
Percentages:	n/a
Media:	visual, paged
Computed value:	specified value

The search for unoccupied space is conducted one Grid Item at a time. First Grid Items which have a constraint on either their grid-row or grid-column are placed, followed by Grid Items which have both a grid-row and grid-column value of 'auto'. If grid-flow is 'rows', Grid Items having an auto value for grid-row are placed before those with an auto value for grid-column. The reverse is true when grid-flow is 'columns'.

In the following example, there are three columns, each auto-sized to their contents. No rows are explicitly defined. The grid-flow property is 'rows' which instructs the grid to search across its three columns starting with the first row, then the next, adding rows as needed until sufficient space is located to accomodate the position of any auto-placed Grid Item. Figure 11 illustrates the result.

<style type="text/css">
    form {
        display: grid;
        grid-columns: "labels" auto "controls" auto "oversized" auto;
        grid-flow: rows
    }
    form > input, form > select {
        /* Place all controls in the "controls" column and automatically find the next available row. */
        grid-column: "controls";
        grid-row: auto
    }
    form > label {
        /* Ensure that label is a valid grid item by setting its display to 'block'. */
        display: block;

        /* Place all labels in the "labels" column and automatically find the next available row. */
        grid-column: "labels";
        grid-row: auto
    }
    
    #department {
        /* Auto place this item in the "oversized" column in the first row where a cell that spans three rows */
        /* won't overlap other explicitly placed items or cells or any items automatically placed prior to    */
        /* this cell. */
        grid-column: "oversized";
        grid-row: auto;
        grid-row-span: 3;
    }

    /* Place all the buttons of the form in the explicitly defined grid cell. */
    #buttons {
        grid-row: auto;

        /* Ensure the button cell spans the entire grid element in the column direction. */
        grid-column: start end;
        grid-column-align: end
    }
</style>
<form action="#">
    <label for="firstname">First name:</label>
    <input type="text" id="firstname" name="firstname" />
    <label for="lastname">Last name:</label>
    <input type="text" id="lastname" name="lastname" />
    <label for="address">Address:</label>
    <input type="text" id="address" name="address" />
    <label for="address2">Address 2:</label>
    <input type="text" id="address2" name="address2" />
    <label for="city">City:</label>
    <input type="text" id="city" name="city" />
    <label for="state">State:</label>
    <select type="text" id="state" name="state">
        <option value="WA">Washington</option>
    </select>
    <label for="zip">Zip:</label>
    <input type="text" id="zip" name="zip" />
    
    <div id="department">
        <label for="department">Department:</label>
        <select id="department" name="department" multiple>
            <option value="finance">Finance</option>
            <option value="humanresources">Human Resources</option>
            <option value="marketing">Marketing</option>
            <option value="payroll">Payroll</option>
            <option value="shipping">Shipping</option>
        </select>
    </div>

   <div id="buttons">
       <button id="cancel">Cancel</button>
       <button id="back">Back</button> 
       <button> id="next">Next</button>
   </div>
</form>

<style type="text/css">
    #grid { display: grid; grid-columns: 1fr 1fr; grid-rows: 1fr 1fr }
    #A { grid-column: 1; grid-row: 1; grid-column-align: start; grid-row-align: start }
    #B { grid-column: 2; grid-row: 2; grid-column-align: end; grid-row-align: end }
</style>

<div id="grid">
    <div id="A">A</div>
    <div id="B">B</div>
</div>

8.1 Grid Item Alignment and Overflow

The values start, end, and center all cause the Grid Item to produce a box sized shrink-to-fit for its cell. If the min-content size of the Grid Item's box is greater than the size of its Cell, it will overflow the bounds of its Cell in a direction determined by its alignment.

A value of stretch causes the Grid Item's box model to shrink or grow to satisfy the following equations:

margin-left + border-left + content-width + border-right + margin-right = <cell width>

margin-top + border-top + content-height + border-bottom + margin-bottom = <cell height>
    		

When stretching a Grid Item box, negative margins can produce a box which is larger than the Grid Item's Cell. Also, stretching can produce a Grid Item box that is smaller than the min-content size of the Grid Item. In such cases the contents of the Grid Item element will overflow its box in accordance with its overflow property.