CSS Grid Layout

2.1. Basic Capabilities of the Grid

Application layout example requiring horizontal and vertical alignment.

As websites evolved from simple documents into complex, interactive applications, tools for document layout, e.g. floats, were not necessarily well suited for application layout. 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 layout capabilities of the Grid address these problems. The Grid provides a mechanism for authors to divide available space for layout 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 area, 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 element 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-definition-rows’ and ‘grid-definition-columns’ properties of the Grid element, and the ‘grid-row-position’, ‘grid-column-position’, ‘grid-row-span’ and ‘grid-column-span’ properties on each Grid item.

<style type="text/css">
<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-column-definition: 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-row-definition: 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-position: 1; grid-row-position: 1 }
    #score    { grid-column-position: 1; grid-row-position: 3 }
    #stats    { grid-column-position: 1; grid-row-position: 2; justify-self: start }
    #board    { grid-column-position: 2; grid-row-position: 1; grid-row-span: 2 }
    #controls { grid-column-position: 2; grid-row-position: 3; align-self: 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 element’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 element’s definition changes.

<style type="text/css">
    @media (orientation: portrait) {
        #grid { 
            display: grid;
            
            /* The rows, columns and areas of the grid are defined visually using the */
            /* grid-template property.  Each string is a row, and each word an area.  */
            /* The number of words in a string determines the number of               */
            /* columns. Note the number of words in each string must be identical.    */
            grid-template: "title stats"
                           "score stats"
                           "board board"
                           "ctrls ctrls";
            
            /* Columns and rows created with the template property can be assigned a sizing */
            /* function with the grid-definition-columns and grid-definition-rows properties. */
            grid-definition-columns: auto minmax(min-content, 1fr); 
            grid-definition-rows: auto auto minmax(min-content, 1fr) auto
        }
    }

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

    /* The grid-area property places a grid item into named region (area) of the grid. */
    #title    { grid-area: title }
    #score    { grid-area: score }
    #stats    { grid-area: stats }
    #board    { grid-area: board }
    #controls { grid-area: ctrls }
</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. 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-definition-columns’ property of the Grid element is updated.

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

        /* The grid-definition-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-definition-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 ending 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-position 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 areas of    */
    /* different shapes like the thumb and track parts  in this example. */
    #lower-label { grid-column: "start" }
    #track       { grid-column: "track-start" "track-end"; align-self: center }
    #upper-label { grid-column: "track-end"; }
    
    /* Fill parts are drawn above the track so set z-index to 5. */
    #lower-fill  { grid-column: "track-start" "fill-split"; align-self: center; z-index: 5 }
    #upper-fill  { grid-column: "fill-split" "track-end"; align-self: center; z-index: 5 }
    
    /* Thumb is the topmost part; assign it the highest z-index value. */
    #thumb       { grid-column: "thumb-start" "thumb-end"; z-index: 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-definition-rows’ and ‘grid-definition-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-definition-columns: 150px 1fr; /* two columns */
        grid-definition-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 Grid line before or after a Grid track definition wherever the desired Grid line would logically exist. A Grid item then uses the Grid lines to determine its position within the Grid by referencing the Grid lines using the properties ‘grid-row-position’ and ‘grid-column-position’.

The following two examples create three column Grid lines and four row Grid lines. The first example demonstrates how an author would position a Grid item using Grid line numbers. The second example uses explicitly named Grid lines.

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

    #item1 { grid-column-position: 2; grid-row-position: 1; grid-row-span: 3 }
</style>

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

Grid Areas are the logical space used to lay out one or more Grid items. Grid areas may be defined explicitly using the ‘grid-template’ property, or implicitly by referencing a region of the Grid using the properties ‘grid-row-position’ and ‘grid-column-position’ on a Grid item.

Whether a Grid area is created explicitly or implicitly, there is no difference in the layout or drawing order of the Grid items which are associated with that Grid area. Grid areas cannot be styled. Only the syntax used to refer to a region of space on the Grid differs between the implicit and explicit approach to provide authors with the tools to best suit their scenarios as illustrated in prior examples.

<style type="text/css">
    /* using the template syntax */
    #grid  { 
        display: grid; 
        grid-template: ". a"
                       "b a"
                       ". a";
        grid-definition-columns: 150px 1fr;
        grid-definition-rows: 50px 1fr 50px
    }

    #item1 { grid-area: a }
    #item2 { grid-area: b }
    #item3 { grid-area: b }

    /* Align items 2 and 3 at different points in the Grid Area "b".  */
    /* By default, Grid Items are stretched to fit their Grid Area    */
    /* and these items would layer one over the other. */
    #item2 { align-self: head }
    #item3 { justify-self: end; align-self: foot }</style>

4.1. Grid Declaration

A Grid element is declared by setting the display property.

grid

A value of grid causes an element to display as a block-level Grid element.

inline-grid

A value of inline-grid causes an element to display as an inline-level Grid element.

The baseline of an inline Grid element is the bottom edge of the margin box.

4.2. Grid Rows and Columns

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-definition-columns: 150px 1fr;
        grid-definition-rows: 50px 1fr 50px
    }
</style>

4.3. Named Grid Lines

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-definition-rows’ or ‘grid-definition-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-definition-columns: "first" "nav" 150px "main" 1fr "last";
        grid-definition-rows: "first" "header" 50px "main" 1fr "footer" 50px "last";
    }
</style>

Consider merging grid line names with grid field names. See: http://lists.w3.org/Archives/Public/www-style/2012Sep/0047.html for proposal.

4.4. Repeating Columns and Rows

If there are large number of columns or rows that are the same or exhibit a recurring pattern, a repeat function 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 column. Note that when the repeat function 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-definition-columns: 10px "content" 250px 10px 250px 10px 250px 10px 250px 10px;
        grid-definition-rows: 1fr;
    }

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

4.5. Grid-definition-columns and Grid-definition-rows Properties

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

<track-list>    => [ [ <string> ]* <track-group> [ <string> ]* ]+ | none
<track-group>   => <track-minmax> | [ repeat( <positive-integer> , [ [ <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 dependent on content, the result is undefined.
• 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)’.
• auto is equivalent to ‘minmax(min-content, max-content)’.

Consider whether having undefined behavior for percentage-sized tracks in content-sized grids is appropriate. CSS2.1 and CSS3 leave percentage width undefined, although there appears to be consistency between browser implementations.

The current property names imply a grid model more akin to tables than a traditional design grid. Consider adjusting terminology to focus on grid lines rather than the spaces between the lines.

The following example:

• Adds one Grid line 100 pixels from the starting edge 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-definition-columns: 100px 1fr max-content minmax(min-content, 1fr) }

Additional examples of valid Grid track definitions:

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

4.5.1. Fraction Values: ‘fr’

Fraction values are new units applicable to the ‘grid-definition-rows’ and ‘grid-definition-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 of those results is treated as the 1fr value, which is then multiplied by each Grid track’s ‘fraction’ to determine its final size.

4.5.2. Used Values for grid-definition-rows and grid-definition-columns

The used size of all Grid tracks as returned for the ‘grid-definition-rows’ and ‘grid-definition-columns’ properties are normalized to pixel values. All Grid tracks are included in the used value reported for ‘grid-definition-rows’ and ‘grid-definition-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-definition-rows’ or ‘grid-definition-columns’ property. The used value includes 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-definition-columns: 
            "a"         auto 
            "b"         minmax(min-content, 1fr) 
            "b" "c" "d" repeat(2, "e" 40px) 
                        repeat(5, auto);
    }
</style>
<div id="grid">
    <div style="grid-column-position:1; width:50px"></div>
    <div style="grid-column-position:9; width:50px"></div>
</div>
<script type="text/javascript">
    // Returns '"a" 50px "b" 320px "c" "d" repeat(2, "e" 40px) repeat(4, 0px) 50px'.
    var gridElement = document.getElementById("grid");
    window.getComputedStyle(gridElement, null).getPropertyValue("grid-definition-columns");
</script>

4.6. Defining Grid Areas with a Template

Grid areas can also be defined explicitly using identifiers via the ‘grid-template’ property. The ‘grid-template’ property provides a visualization of the Grid element’s structure while simultaneously defining the Grid areas which can be used to position and size the child elements of the Grid.

A row is created for every separate string listed for the grid-template property, and a column is created for each identifier or period in each string. Note that all strings must have the same number of columns. A period represents an unnamed area in the grid element that cannot be used to position or size a grid item. An identifier creates a named grid area that can be used to position and size grid items. No two grid areas may have the same identifier. Duplicate identifiers define a grid area which spans multiple tracks, and must therefore be geometrically adjacent to each other, forming a rectangular shape. A declaration which fails to meet these criteria will be dropped.

In the following example, a ‘grid-template’ property is used to create a page layout where areas are defined for header content head, navigational content nav, footer content foot, and main content main. Accordingly, the template creates three rows, two columns, and four areas. The head area spans both columns and the first row of the grid. Columns are the tracks that run in the inline direction and rows are the tracks running in the direction of block progression. See Grid Writing Modes for more details.

The ‘grid-area’ property, defined in further detail below, is specified on Grid items to position the Grid item inside an explicitly named Grid area. The ‘grid-area’ property, defined in further detail below, is specified on Grid items to position the Grid item inside an explicitly named Grid area.

<style type="text/css">
    #grid {
        display: grid;
        grid-template: "head head"
                       "nav  main"
                       "foot ."
    }
    #grid > a {
        grid-area: "nav";
    }
</style>

6.2. Grid shorthand properties

For all shorthand properties below, specifying an <identifier> sets the values of all longhand properties associated with the shorthand property to the values associated with the <identifier> .

The ‘grid-column-position’ and ‘grid-column-span’ properties can be simultaneously specified with the ‘grid-column’ shorthand property. Likewise, the ‘grid-row-position’ and ‘grid-row-span’ properties can be simultaneously specified with the ‘grid-row’ shorthand property. Note that the first input refers to the starting grid line, while the second, optional input refers to the span of the grid item if an integer, and the ending grid line if a string. Note also that the second input cannot be a string if the first input is ‘auto’.

The ‘grid-column-position’ and ‘grid-row-position’ of an item can be simultaneously specified with the ‘grid-position’ shorthand property. The starting grid line for the row and column, respectively, may be specified by an integer, a string, or an ‘auto’ value which will be resolved by the automatic placement algorithm described below.

The ‘grid-column-span’ and ‘grid-row-span’ properties can be simultaneously specified with the ‘grid-span’ shorthand property. When only one integer is specified, the row and column spans are both set to the integer value; when two integer values are specified, the row and column spans are set, respectively.

The ‘grid-area’ property is a shorthand for the ‘grid-column-position’, ‘grid-row-position’, ‘grid-column-span’, and ‘grid-row-span’ properties. The first two inputs specify the row and column positions, respectively, and may be set with integers or strings, or auto keywords which will be resolved with the auto-positioning algorithm.

The next input may be a single integer which simultaneously specifies both the row and column spans. Alternatively, the row and column positions may be followed by two inputs, which may be either integers or strings and specify the row and column spans, respectively. An integer input sets the track span of a grid item, while a string input sets the ending grid line of the grid item. Note that strings cannot be used to specify the ending grid line if the corresponding row or column position of the grid item has been set to ‘auto’.

Note that the use of a shorthand property resets all properties covered by that property to their initial value, except for the properties explicitly set by the shorthand property.

<style type="text/css">
#item {
    /* Initial row and column positions are auto */
    /* Initial row and column spans are 1 */

    /* The following property will set row position to 3 */
    grid-row-position: 3;
	
    /* The following property will set row position to 3 */
    /* and reset the row-span property to 1 */
    grid-row: 3;
	
    /* The following property will set column position to 3 */
    /* And column span to 2 */
    grid-column: 3 2;

    /* The following property will set row span to 2 */
    /* And column span to 3 */
    /* Note that any previously set position is unaffected */
    grid-span: 2 3;

    /* The following property will set row and column position to 4 */
    grid-position: 4 4;
	
    /* The following property will set row and column position to 2 */
    /* And set row and column span to 5 */
    grid-area: 2 2 5;

    /* The following property will set row and column position to 4 */
    /* And reset row and column span to 1 */
    grid-area: 4 4;

}
</style>

6.3. Anonymous Grid Areas

Each Grid item is contained by a Grid area, i.e. the Grid area serves as the containing block for the Grid item. The dimensions of an anonymous Grid area are determined by naming the starting and ending Grid lines using the ‘grid-row’ and ‘grid-column’ properties on the Grid item which the Grid area surrounds. The starting and ending lines may be referred to by a string name, if one was defined by the author, or the Grid line’s number.

The following example positions the first Grid item to cover the first two rows and columns of the Grid element. The second Grid item is positioned to cover the first row. Note that when only the starting Grid line of the ‘grid-row-position’ or ‘grid-column-position’ properties are specified for the Grid area, that the Grid area is assumed to extend to the subsequent Grid line.

<style type="text/css">
    /* covers the Grid element’s content box */
    #item1 {
        grid-row: 1 3;
        grid-column: 1 3;
    }
 
    /* covers the first row */
    #item2 {
        grid-row-position: 1; /* extends to row Grid Line 2 */
        grid-column-position: 1 3;
    }
</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-definition-rows: "header" auto "main" 1fr "footer" auto;
        grid-definition-columns: 1fr;
    }
 
    #header { grid-row-position: "header"; grid-column-position: start }
    #main   { grid-row-position: "main"; grid-column-position: start }
    #footer { grid-row-position: "footer"; grid-column-position: start }

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

6.4. Implicit Columns and Rows

Grid line numbers referred to by ‘grid-row-position’ or ‘grid-column-position’ properties on a Grid item are not required to refer to a Grid line that was defined using the ‘grid-definition-columns’ or ‘grid-definition-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 also that quoted Grid line names that don't resolve to an explicitly defined Grid line are treated as though the author specified the first 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-definition-columns: 20px; grid-definition-rows: 20px }
    #A { grid-column-position: 1; grid-row-position: 1; justify-self: start; align-self: head }
    #B { grid-column-position: 5; grid-row-position: 1; grid-row-span: 2; }
    #C { grid-column-position: 1; grid-row-position: 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>

6.5. Automatic Placement of Grid Items

Grid items can be automatically placed into an unoccupied space of the Grid. The ‘grid-auto-flow’ property controls the direction in which the search for unoccupied space takes place, and whether rows or columns are added as needed to accommodate the content.

A ‘grid-auto-flow’ value of ‘rows’ will place grid items in row-major order by searching across columns and then rows for unoccupied space, and will create additional rows as needed to accommodate content grid items. Similarly, a ‘grid-auto-flow’ value of ‘columns’ will place grid items in column-major order, creating new columns as needed.

The search for unoccupied space is conducted one Grid item at a time. Grid items which have a constraint on their major axis are placed first (the major axis being rows for a ‘grid-auto-flow’ value of ‘rows’ and columns for a ‘grid-auto-flow’ value of ‘columns’). Note that constraint means a value other than ‘auto’ for the applicable ‘grid-row-position’ or ‘grid-column-position’ property. After this step, the number of minor-axis tracks must be determined for conducting the second phase of the auto-placement algorithm.

For the purposes of operating the auto-placement algorithm on items that have no constraint on their major axis, the number of tracks on the minor axis must be determined. The number of tracks will be the largest number implied by the ‘grid-template’ property, the ‘grid-row-definition’ and ‘grid-column-definition’ properties, the position and size of any explicitly-defined grid items, or any minor-axis ‘auto’ values resolved by the previous phase of the algorithm. Note that any ‘auto’ values still unresolved will be treated as 1 for the purposes of this determination.

To position all remaining items that have ‘auto’ values left unresolved, starting at position 1,1 on the grid, determine the minimum value along the minor axis that a grid item can be placed at while not overlapping the grid area of any other grid item, or exceeding the previously determined maximum track along the minor axis. If no such value can be found along the minor axis, increment the major axis value, creating new major axis tracks as necessary.

In the following example, there are three columns, each auto-sized to their contents. No rows are explicitly defined. The ‘grid-auto-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 accommodate the position of any auto-placed Grid item. Figure 11 illustrates the result.

<style type="text/css">
    form {
        display: grid;
        grid-definition-columns: "labels" auto "controls" auto "oversized" auto;
        grid-auto-flow: rows
    }
    form > input, form > select {
        /* Place all controls in the "controls" column and automatically find the */
        /* next available row. */
        grid-column-position: "controls";
        grid-row-position: auto
    }
    form > label {
        /* Place all labels in the "labels" column and automatically find the next
        /* available row. */
        grid-column-position: "labels";
        grid-row-position: auto
    }
    
    #department {
        /* Auto place this item in the "oversized" column in the first row where an area that  */
        /* spans three rows won't overlap other explicitly placed items or areas or any items */
        /* automatically placed prior to this area. */
        grid-column-position: "oversized";
        grid-row-position: auto;
        grid-row-span: 3;
    }

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

        /* Ensure the button area spans the entire grid element in the column direction. */
        grid-column-span: 3;
        justify-self: 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>
        </select>
    </div>

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

7.1. Impact of Alignment on Grid Item Size

The values ‘start’, ‘end’, and ‘center’ all cause the Grid item to produce a box sized shrink-to-fit for its area in accordance with the CSS3 Box Model. Note that percentage lengths specified on a Grid item resolve against the dimensions of the Grid area (i.e. the Grid area serves as the containing block for the Grid item). Percentages specified for margin-top, padding-top, margin-bottom, and padding-bottom resolve against the height of the Grid area, rather than the width of the Grid area.

A value of ‘stretch’ causes the Grid item’s width to shrink or grow in accordance with the equation for calculating width described in section 9.3 of the CSS3 Box Model, with the following exception: the paragraph in 9.3 which describes the behavior of items that are larger than their containing block is not applicable for the purpose of calculating the size of Grid items. Note that this calculation is symmetric for both the width and height of the Grid item.

9.1. Definition of Terms for use in Calculating Grid Track Sizes

AvailableSpace

The Grid element’s content box size in the applicable dimension.

MaxTrackSizingFunction

One of the <track-breadth> sizing functions assigned as the maximum breadth of a Grid track.

MinTrackSizingFunction

One of the <track-breadth> sizing functions assigned as the minimum breadth of a Grid track.

RemainingSpace

The max of zero and the AvailableSpace less the sum of all Grid track UsedBreadth values. This is undefined if AvailableSpace is undefined (i.e. the Grid element is shrink-to-fit or the height is auto.)

SpanCount

The number of Grid tracks crossed by a Grid item in the applicable dimension.

9.2. Grid Track Sizing Algorithm

1. Call ComputedUsedBreadthOfGridTracks for Grid Columns to resolve their logical width.
2. Call ComputedUsedBreadthOfGridTracks for Grid Rows to resolve their logical height. The logical width of Grid Columns from the prior step is used in the formatting of Grid items in content-sized Grid Rows to determine their required height.
3. If the minimum content size of any Grid item has changed based on available height for the Grid item as computed in step 2, adjust the min content size of the Grid item and restart the Grid track Sizing algorithm (once only).

ComputeUsedBreadthOfGridTracks

This is the core Grid track sizing algorithm. It is run for Grid columns and Grid rows. The goal of the function is to ensure:

1. That each Grid track satisfies its MinTrackSizingFunction
2. That each Grid track grows from the breadth which satisfied its MinTrackSizingFunction to a breadth which satisfies its MaxTrackSizingFunction, subject to RemainingSpace.

For the purposes of resolving the breadth that satisfies the MinTrackSizingFunction and MaxTrackSizingFunction, each Grid track falls into one of three categories:

1. A percentage or length value which can be trivially resolved.
2. A min-content or max-content value which will be resolved based on the measurements of the Grid items which cover the Grid track.
3. A fraction value which can only be resolved after determining the RemainingSpace in the Grid element’s content box after having considered all contributions from the prior two categories of Grid tracks.

The breadths which satisfy MinTrackSizingFunctions and MaxTrackSizingFunctions for the first category of Grid tracks are resolved in step 1 during Grid track variable initialization. The breadths which satisfy the MinTrackSizingFunctions and the MaxTrackSizingFunctions for the second category of content-sized Grid tracks are resolved in step 2. At the end of step 2, the first goal of ComputeUsedBreadthOfGridTracks function has been satisfied: the UsedBreadth variable of each GridTrack now satisfies its MinTrackSizingFunction. The MaxBreadth variable for each Grid track now contains the resolved value for its MaxTrackSizingFunction.

In step 3, the second goal of this function is satisfied as each (non-fraction-sized) Grid track attempts to grow from the UsedBreadth value to the MaxBreadth value, subject to RemainingSpace.

Finally in step 4, the third category of fraction-sized Grid tracks can be resolved using what is now the RemainingSpace having updated the UsedBreadth of each Grid track at the end of step 3.

Inputs

• GridTracks: The set of Grid tracks for which the Grid track variable UsedBreadth will be computed.

Algorithm

1. Initialize per Grid track variables
   1. For each Grid track t in GridTracks
      1. If t.MinTrackSizingFunction is a percentage or length, then t.UsedBreadth = resolved length
      2. If t.MinTrackSizingFunction is min-content, max-content, or a fraction, then t.UsedBreadth = 0
      3. If t.MaxTrackSizingFunction is percentage or length, then t.MaxBreadth = resolved length
         1. If the resolved length of the MaxTrackSizingFunction is less than the MinTrackSizingFunction, t.MaxBreadth = t.UsedBreadth.
      4. If t.MaxTrackSizingFunction is min-content, or max-content, then t.MaxBreadth = Infinity
      5. If t.MaxTrackSizingFunction is a fraction, then t.MaxBreadth = t.UsedBreadth
      6. t.SpanGroupInWhichMaxBreadthWasMadeFinite = null
2. Resolve content-based TrackSizingFunctions
   1. Call ResolveContentBasedTrackSizingFunctions, with arguments:
      • GridItems: All Grid items.
3. Grow all Grid tracks in GridTracks from their UsedBreadth up to their MaxBreadth value until RemainingSpace is exhausted.
   1. If RemainingSpace is defined
      1. Iterate over all GridTracks and assign UsedBreadth to UpdatedTrackBreadth
      2. Call DistributeSpaceToTracks, with arguments:
         • SpaceToDistribute: RemainingSpace
         • TrackGrowthConstraint: A function which given a Grid track returns its MaxBreadth.
         • TracksForGrowth: All Grid tracks
         • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: The empty set.
         • CurrentBreadth: A function which given a Grid track returns the UsedBreadth.
      3. Iterate over all GridTracks and assign UpdatedTrackBreadth to UsedBreadth
   2. Else
      1. For each Grid track t in GridTracks
         1. t.UsedBreadth = t.MaxBreadth
4. Grow all Grid tracks having a fraction as the MaxTrackSizingFunction
   1. normalizedFractionBreadth = 0
   2. If RemainingSpace is defined
      1. normalizedFractionBreadth = Call ComputeNormalizedFractionBreadth, with arguments:
         • GridTracks: GridTracks
         • SpaceToFill: AvailableSpace
   3. Else
      1. For each Grid track t in GridTracks having a fraction as the MaxTrackSizingFunction
         1. normalizedFractionBreadth = Max( normalizedFractionBreadth, t.UsedBreadth / t.MaxTrackSizingFunction.FractionValue )
      2. For each Grid item i, which crosses a set of GridTracks s
         1. itemNormalizedFractionBreadth = Call ComputeNormalizedFractionBreadth, with arguments:
            • GridTracks: s
            • SpaceToFill: max-content size of i
         2. normalizedFractionBreadth = Max( normalizedFractionBreadth, itemNormalizedFractionBreadth )
   4. For each Grid track t in GridTracks
      1. t.UsedBreadth = Max( t.UsedBreadth, normalizedFractionBreadth * t.MaxTrackSizingFunction.FractionValue )

ResolveContentBasedTrackSizingFunctions

The purpose of this function is to resolve the contribution that each Grid item makes to any min-content or max-content TrackSizingFunctions for the Grid tracks it covers. There are four permutations: min-content or max-content in either the MinTrackSizingFunction or MaxTrackSizingFunction. MinTrackSizingFunctions are resolved before MaxTrackSizingFunctions, and min-content contributions are resolved before max-content contributions. Note that when resolving min-content contributions they may grow tracks that have either min-content or max-content keywords, as seen in 3.a.i and 3.b.i below.

Currently this algorithm embodies several heuristics which regulate the growth of spanning Grid items to accommodate certain use cases. (E.g. the game example in Figures 2 and 3 above.) These heuristics should be a normative part of this specification to ensure interoperability. To the extent additional use cases can be identified that cannot be satisfied by following the current heuristics, the normative algorithm can be updated, or additional mechanisms can be introduced for fine-grained control of content-based TrackSizingFunction.

Inputs

• GridItems: The set of Grid items for which min-content or max-content keywords should be resolved.

Algorithm

1. Filter all Grid items into a set g, such that each Grid item has either a SpanCount of 1 or does not cross a fraction-sized Grid track
2. Group all Grid items in set g by their SpanCount ascending
3. For each group of Grid items
   1. Resolve content-based MinTrackSizingFunctions
      1. Call ResolveContentBasedTrackSizingFunctionsForItems, with arguments:
         • GridItems: All Grid items in the current group.
         • AdditionalSpaceRequiredByItem: A function which given a Grid item returns the min-content size of that Grid item less the summed UsedBreadth of all Grid tracks it covers.
         • TrackGrowthConstraint: A function which given a Grid track returns its MaxBreadth.
         • TracksForGrowth: A function which given a Grid item returns the set of Grid tracks covered by that Grid item that have a min-content or max-content MinTrackSizingFunction.
         • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: A function which given a set of Grid tracks returns the subset of Grid tracks having a min-content or max-content MaxTrackSizingFunction. If that set is the empty set, return the input set instead.
         • Accumulator: A function which given a Grid track returns a reference to its UsedBreadth variable.
      2. Call ResolveContentBasedTrackSizingFunctionsForItems, with arguments:
         • GridItems: All Grid items in the current group.
         • AdditionalSpaceRequiredByItem: A function which given a Grid item returns the max-content size of that Grid item less the summed UsedBreadth of all Grid tracks it covers.
         • TrackGrowthConstraint: A function which given a Grid track returns its MaxBreadth.
         • TracksForGrowth: A function which given a Grid item returns the set of Grid tracks covered by that Grid item that have a max-content MinTrackSizingFunction.
         • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: A function which given a set of Grid tracks returns the subset of Grid tracks having a max-content MaxTrackSizingFunction. If that set is the empty set, return the input set instead.
         • Accumulator: A function which given a Grid track returns a reference to its UsedBreadth variable.
   2. Resolve content-based MaxTrackSizingFunctions
      1. Call ResolveContentBasedTrackSizingFunctionsForItems, with arguments:
         • GridItems: All Grid items in the current group.
         • AdditionalSpaceRequiredByItem: A function which given a Grid item returns the min-content size of that Grid item less the summed MaxBreadth (unless the MaxBreadth is infinite, in which case use the UsedBreadth) of all Grid tracks it covers.
         • TrackGrowthConstraint: A function which given a Grid track returns its MaxBreadth.
         • TracksForGrowth: A function which given a Grid item returns the set of Grid tracks covered by that Grid item that have a min-content or max-content MaxTrackSizingFunction.
         • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: The identity function.
         • Accumulator: A function which given a Grid track returns a reference to its MaxBreadth variable.
      2. Call ResolveContentBasedTrackSizingFunctionsForItems, with arguments:
         • GridItems: All Grid items in the current group.
         • AdditionalSpaceRequiredByItem: A function which given a Grid item returns the max-content size of that Grid item less the summed MaxBreadth (unless the MaxBreadth is infinite, in which case use the UsedBreadth) of all Grid tracks it covers.
         • TrackGrowthConstraint: A function which given a Grid track returns infinity if the Grid track’s SpanGroupInWhichMaxBreadthWasMadeFinite is equal to the current group; otherwise return the Grid track’s MaxBreadth.
         • TracksForGrowth: A function which given a Grid item returns the set of Grid tracks covered by that Grid item that have a max-content MaxTrackSizingFunction.
         • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: The identity function.
         • Accumulator: A function which given a Grid track returns a reference to its MaxBreadth variable.
4. For each Grid track t from the set of all Grid tracks
   1. If t.MaxBreadth == infinity then t.MaxBreadth = t.UsedBreadth

ResolveContentBasedTrackSizingFunctionsForItems

The above function, ResolveContentBasedTrackSizingFunctions, groups Grid items based on the number of Grid tracks each Grid item spanned. ResolveContentBasedTrackSizingFunctionsForItems, below, then calls DistributeSpaceToTracks for each Grid item in the group to determine how much each Grid item needs to grow the Grid tracks that it covers. The maximum contribution made by any Grid item is accumulated into a temporary, per-Grid track variable, and at the end of the group, the space is recorded into a final Grid track variable as determined by the Accumulator function.

Inputs

• GridItems: The set of Grid items which will contribute to the growth of Grid tracks.
• AdditionalSpaceRequiredByItem: A function which returns the difference between either the min-content or max-content for the Grid item and the space already allocated to the Grid tracks covered by the Grid item in earlier phases of the algorithm.
• TrackGrowthConstraint: A function which given a Grid track returns a value that limits the amount by which it may be grown by the Grid items which cover it.
• TracksForGrowth: A function which given a Grid item identifies the set of Grid tracks to be grown in this phase of the algorithm.
• SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: A function which identifies a subset of Grid tracks from TracksForGrowth that may be grown in this phase of the algorithm after all Grid tracks in the TracksForGrowth set have already grown to their TrackGrowthConstraint.
• Accumulator: A function which given a Grid track returns the variable used to accumulate the results of the UpdatedTrackBreadth from DistributeSpaceToTracks.

Algorithm

1. For each Grid track t
   1. t.UpdatedTrackBreadth = Accumulator( t )
2. For each Grid item i in GridItems
   1. Call DistributeSpaceToTracks, with arguments:
      • SpaceToDistribute: AdditionalSpaceRequiredByItem( i )
      • TrackGrowthConstraint: TrackGrowthConstraint
      • TracksForGrowth: TracksForGrowth( i )
      • SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: SubsetOfTracksForGrowthBeyondTrackGrowthConstraint( TracksForGrowth( i ) )
      • CurrentBreadth: A function which given a Grid track returns the UsedBreadth of the Grid track if Accumulator returns infinity; otherwise the value of the Accumulator is returned.
3. For each Grid track t
   1. If Accumulator( t ) == infinity and t.UpdatedTrackBreadth != infinity
      1. t.SpanGroupInWhichMaxBreadthWasMadeFinite = GridItems
   2. Accumulator( t ) = t.UpdatedTrackBreadth

DistributeSpaceToTracks

Ensures that for each Grid track in RecipientTracks, a value will be computed, UpdatedTrackBreadth, that represents the Grid track’s share of SpaceToDistribute.

There are two parts to this function. The first for loop in step 2 is giving each Grid track an equal share of the space, but without exceeding their TrackGrowthConstraint values. Because there are different MaxBreadths assigned to the different Grid tracks, the first loop can result in their uneven growth.

If the first loop completes having grown every Grid track to its TrackGrowthConstraint, and there is still SpaceToDistribute, then SubsetOfTracksForGrowthBeyondTrackGrowthConstraint are further grown equally until SpaceToDistribute is exhausted.

Note that Grid tracks considered by this function may have a TrackGrowthConstraint equal to Infinity, which signals that these tracks have not yet been grown by a Grid item. These tracks can therefore be grown without exceeding the TrackGrowthConstraint of the track. By only growing tracks up to their TrackGrowthConstraint value, we can ensure that the grid remains "tight" - that is, that track breadth is as close to the content size of the Grid items inside as possible. Only once all Grid tracks have a CurrentBreadth equal to a TrackGrowthConstraint do we move to the second for loop and grow tracks further, thereby making the Grid element less tight.

Inputs

• SpaceToDistribute: A length to be distributed among the supplied set of Grid tracks.
• TrackGrowthConstraint: A function which given a Grid track returns the maximum breadth of the track, unless the track is in the SubsetOfTracksForGrowthBeyoundTrackGrowthConstraint.
• TracksForGrowth: A set of Grid tracks to be grown up to their TrackGrowthConstraint while SpaceToDistribute remains.
• SubsetOfTracksForGrowthBeyondTrackGrowthConstraint: A subset of Grid tracks from TracksForGrowth that may be grown beyond their TrackGrowthConstraint after all Grid tracks in the TracksForGrowth set have already grown to their TrackGrowthConstraint if there is remaining SpaceToDistribute.
• CurrentBreadth: A function which given a Grid track returns a value to use as a starting point from which to grow this track.

Algorithm

1. Sort TracksForGrowth by TrackGrowthConstraint( t ) - CurrentBreadth( t ) ascending (where t is an element in the RecipientTrack set).
2. For i = 0 to TracksForGrowth.length - 1
   1. t = TracksForGrowth[i]
   2. share = Min ((SpaceToDistribute / ( TracksForGrowth.length - i )), (TrackGrowthConstraint( t ) - CurrentBreadth( t )))
   3. t.TempBreadth = CurrentBreadth( t ) + share
   4. SpaceToDistribute -= share
3. If SpaceToDistribute > 0
   1. Let tracks = SubsetOfTracksForGrowthBeyondTrackGrowthConstraint( TracksForGrowth )
   2. For i = 0 to tracks.length - 1
      1. t = tracks[i]
      2. share = SpaceToDistribute / ( tracks.length - i )
      3. t.TempBreadth += share
      4. SpaceToDistribute -= share
4. For each Grid track t in TracksForGrowth
   1. If t.UpdatedTrackBreadth == infinity
      1. t.UpdatedTrackBreadth = t.TempBreadth
   2. Else
      1. t.UpdatedTrackBreadth = Max( t.UpdatedTrackBreadth, t.TempBreadth )

CalculateNormalizedFractionBreadth

This method computes a ‘1fr’ value, referred to as the NormalizedFractionBreadth, for a set of Grid tracks. The value computed will ensure that when the NormalizedFractionBreadth is multiplied by the fractions associated with GridTracks, that the UsedBreadths of GridTracks will increase by an amount equal to the maximum of zero and the specified SpaceToFill less the sum of the current UsedBreadths.

Inputs

• GridTracks: The set of Grid tracks whose fraction sizing functions are considered for the purposes of a computing a NormalizedFractionBreadth that will cause GridTracks to fill SpaceToFill.
• SpaceToFill: The space to be filled by GridTracks.

Returns

• The 1fr value required by GridTracks to fill SpaceToFill.

Algorithm

1. allocatedSpace = the sum of the UsedBreadth for each Grid track in GridTracks
2. fractionTracks = the subset of GridTracks whose MaxTrackSizingFunction is a fraction
3. For each Grid track t in fractionTracks
   1. t.NormalizedFractionValue = t.UsedBreadth / t.MaxTrackSizingFunction.FractionValue
4. Sort fractionTracks by their NormalizedFractionValue ascending
5. spaceNeededFromFractionTracks = SpaceToFill - allocatedSpace
6. currentBandFractionBreadth = accumulatedFractions = 0
7. For each Grid track t in fractionTracks
   1. If t.NormalizedFractionValue > currentBandFractionBreadth
      1. If t.NormalizedFractionValue * accumulatedFractions > spaceNeededFromFractionTracks then break from for loop
      2. currentBandFractionBreadth = t.NormalizedFractionValue
   2. accumulatedFractions += t.MaxTrackSizingFunction.FractionValue
   3. spaceNeededFromFractionTracks += t.UsedBreadth
8. return spaceNeededFromFractionTracks / accumulatedFractions