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<pre class='metadata'>
Title: CSS Grid Layout Module Level 3
Shortname: css-grid
Level: 3
Status: ED
Work Status: Exploring
Group: csswg
ED: https://drafts.csswg.org/css-grid-3/
Editor: Tab Atkins Jr., Google, http://www.xanthir.com/contact/, w3cid 42199
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Mats Palmgren, Mozilla, mailto:mats@mozilla.com
Editor: Jen Simmons, Apple, http://jensimmons.com/
Editor: Brandon Stewart, Apple, https://brandonstewart.net
Abstract: This module introduces masonry layout as an additional layout mode for <a href="https://www.w3.org/TR/css-grid-2/">CSS Grid</a> containers.
WPT Path Prefix: css/css-grid/masonry/tentative/
Markup Shorthands: css yes
Status Text: <strong>This specification represents two variations on the proposal for masonry layout. Feedback on the alternatives is welcome.
</pre>
<style>
:is(section, p).option {
border-right: 1em double;
padding-right: 1em;
}
:is(section, p).option.grid {
border-right-color: #be2596;
}
:is(section, p).option.masonry {
border-right-color: #96be25;
}
:is(span, a).option {
font-weight: bold;
}
:is(span, a).option * {
color: inherit;
}
:is(span, a).option.grid {
color: #be2596;
}
:is(span, a).option.masonry {
color: #658118
}
</style>
<h2 id='intro'>
Introduction</h2>
<em>This section is not normative.</em>
Grid Layout is a layout model for CSS
that has powerful abilities to control the sizing and positioning
of boxes and their contents.
Grid Layout is optimized for 2-dimensional layouts:
those in which alignment of content is desired in both dimensions.
<figure>
<img src="images/grid-layout.png"
alt="An example of grid layout:
two rows of items,
the first being four items — the last of which spans both rows,
and the second being two items —
the first of which spans the first two columns —
plus the spanned item from the first row.">
<figcaption>Representative Grid layout example</figcaption>
</figure>
Although many layouts can be expressed with regular Grid Layout,
restricting items into a grid in both axes also makes it impossible
to express some common layouts on the Web.
This module defines a layout system that removes that restriction
so that items can be placed into Grid-like tracks in just one of the axes,
while stacking them one after another in the other axis.
Items are placed into the column (or row) with the most remaining space
based on the layout size of the items placed so far.
This module also extends <a href="https://www.w3.org/TR/css-grid-2/">CSS Grid</a>
with this new grid item placement strategy
and <a href="https://drafts.csswg.org/css-align">CSS Box Alignment</a> with new alignment features.
<h3 id='background'>
Background and Motivation</h3>
[=Masonry layout=], sometimes also called “waterfall layout”,
is a common Web design pattern where a number of items--
commonly images or short article summaries--
are placed one by one into columns
in a way that loosely resembles stone masonry.
Unlike [[CSS-MULTICOL-1|multi-column layout]],
where content is placed vertically in the first column
until it must spills over to the second column,
[=masonry layout=] selects a column for each new item
such that it is generally closer to the top of the layout than items placed later.
<div class="example">
The Pinterest search results page exemplifies this layout:
<figure>
<img src="images/pinterest.png"
alt="An example of masonry layout:
four columns of items,
each item is placed into the column with the smallest height so far.">
<figcaption>Representative masonry layout example</figcaption>
</figure>
Here, each item has a different height
(depending on the content and the width of the column),
and inspecting the DOM reveals
(as the visual content itself gives no indication of ordering)
that each item has been placed into the column with the smallest height so far.
</div>
This layout superficially looks similar to multi-column layout;
but it has the advantage that scrolling down
will naturally lead to "later" items in the layout
(that is, those less relevant in the search results).
It's not possible to achieve this layout using earlier CSS layout models,
unless you know up-front how tall each item will be,
or use JavaScript for content measurement or placement.
<h3 id="alternative-proposals">
Alternative Syntax Proposals</h3>
<div class="issue">
The masonry layout feature of this specification is drafted with two different syntaxes
to promote debate and discussion of these two alternatives.
<dl>
<dt><dfn><span class="option grid">Grid-integrated Option</span></dfn>
<dd>
In this syntax model, [=masonry layout=] is integrated into the [=grid layout=] model,
re-using all of the 'grid-*' properties.
<dt><dfn><span class="option masonry">Grid-independent Option</span></dfn>
<dd>
In this syntax model, [=masonry layout=] is its own 'display' type,
and has its own parallel set of 'masonry-*' properties.
</dl>
</div>
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h2 id='masonry-model'>
Masonry Layout Model</h2>
<dfn>Masonry layout</dfn>
lays out items into pre-defined tracks similar to [=grid layout=] in one axis
(called the <dfn local-lt="grid-axis">grid axis</dfn>),
but flows them freely similar to [=flex layout=] in the other
(called the <dfn local-lt="masonry-axis">masonry axis</dfn>).
Similar to [=grid layout=] and unlike [=flex layout=],
[=masonry layout=]’s auto-placement
distributes items across the tracks to keep the lengths of those tracks
as similar as possible.
[=Grid items=] are formed and [=blockified=]
exactly the same as in a regular [=grid container=].
All CSS properties work the same as in a regular [=grid container=]
unless otherwise specified by this specification.
For example, ''order'' can be used to specify a different layout order for the items.
Note: Subgrid items are supported,
but subgridding only occurs in the [=grid axis=];
see [[#subgrids]] for details.
A <dfn>masonry container</dfn> is a box whose contents participate in [=masonry layout=].
A [=masonry container=] is a <dfn export>column masonry container</dfn>
if its [=masonry axis=] is the [=block axis=],
or a <dfn export>row masonry container</dfn>
if its [=masonry axis=] is the [=inline axis=].
<table class=data>
<caption>Establishing Masonry Containers</caption>
<thead>
<th>
<th><a href="#masonry-model-grid-option" class="option grid">Grid-integrated Syntax</a>
<th><a href="#masonry-model-masonry-option" class="option masonry">Grid-independent Syntax</a>
<th>Description
<tbody>
<tr>
<th style="writing-mode: vertical-rl; writing-mode: sideways-lr">
Column Masonry
<td>
<pre>
display: grid;
grid-template-columns: 1fr 2fr 3fr;
grid-template-rows: masonry;
</pre>
<td>
<pre>
display: masonry;
masonry-tracks: 1fr 2fr 3fr;
masonry-direction: column;
</pre>
<td>
<img src="images/masonry-columns.png"
alt="Column masonry lays out items in columns,
but ordered across the columns,
placing each item in the then-shortest column.">
<tr>
<th style="writing-mode: vertical-rl; writing-mode: sideways-lr">
Row Masonry
<td>
<pre>
display: grid;
grid-template-rows: 1fr 2fr 3fr;
grid-template-columns: masonry;
</pre>
<td>
<pre>
display: masonry;
masonry-tracks: 1fr 2fr 3fr;
masonry-direction: row;
</pre>
<td>
<img src="images/masonry-rows.png"
alt="Row masonry lays out items in rows,
but ordered down across the rows,
placing each item in the then-shortest row.">
</table>
<section class="option grid">
<h3 id="masonry-model-grid-option">
<span class="option grid">Grid-integrated Option:</span> Establishing Masonry Layout</h3>
<h4 id="grid-template-masonry">
Indicating the Masonry Axis: the ''grid-template-columns/masonry'' keyword for 'grid-template-columns'/'grid-template-rows'</h3>
<pre class='propdef partial'>
Name: grid-template-columns, grid-template-rows
New values: masonry
Initial: none
Applies to: [=grid containers=]
Inherited: no
Percentages: refer to corresponding dimension of the content area
Computed value: the keyword ''grid-template-columns/none'' or the keyword ''grid-template-columns/masonry'' or a [[computed track list]]
Animation type: see [[css-grid-2#track-sizing|CSS Grid Level 2]]
</pre>
[=Masonry layout=] can be applied to [=grid containers=]
by specifying the value ''grid-template-columns/masonry'' for one of its axes,
defining it as the [=masonry axis=].
Such [=grid containers=] are called [=masonry containers=].
If ''grid-template-columns/masonry'' is specified
for both ''grid-template-columns'' and ''grid-template-rows'',
then the [=used value=] for ''grid-template-columns'' is ''grid-template-columns/none'',
and thus the [=inline axis=] will be the [=grid axis=].
<h4 id="grid-auto-flow">
Auto Flow Directions: the 'grid-auto-flow' property</h4>
<pre class='propdef'>
Name: grid-auto-flow
Value: [ row | column | row-reverse | column-reverse ] || dense || wrap-reverse
Initial: row
Applies to: <a>grid containers</a>
Inherited: no
Percentages: n/a
Computed value: specified keyword(s)
Animation type: discrete
</pre>
<p dfn-for=grid-auto-flow>This specification extends the <a href="https://www.w3.org/TR/css-grid-2/#grid-auto-flow-property">grid-auto-flow</a> property to add
<dfn value>row-reverse</dfn>,
<dfn value>column-reverse</dfn>,
and
<dfn value>wrap-reverse</dfn>,
which reverse the directions of the [=auto-placement algorithm=].
In [=masonry layout=], the flow axis specified by ''grid-auto-flow'' is ignored:
items are always placed by filling across the [=grid axis=].
ISSUE: Should that be the case, or should we follow the 'grid-auto-flow' axis always
(so it would fill similar to flexbox, but with explicitly-sized tracks)?
ISSUE(9326):
Should <css>dense</css> packing apply to masonry?
It's much more expensive in masonry,
as you have to lay out the element in every possible gap spot
to see if it's short enough to fit;
Grid gets to just juggle some integers.
</section>
<section class="option masonry">
<h3 id="masonry-model-masonry-option">
<span class="option masonry">Grid-independent Option:</span> Establishing Masonry Layout</h3>
<h4 id="masonry-display">
Establishing Masonry Containers: the ''masonry'' and ''inline-masonry'' 'display' values</h4>
<pre class="propdef partial">
Name: display
New values: masonry | inline-masonry
</pre>
<dl dfn-type=value dfn-for=display>
: <dfn>masonry</dfn>
:: This value causes an element to generate a [=masonry container=] box
that is [=block-level=] when placed in [=flow layout=].
: <dfn>inline-masonry</dfn>
:: This value causes an element to generate a [=masonry container=] box
that is [=inline-level=] when placed in [=flow layout=].
</dl>
<h4 id=masonry-direction>
Masonry Track Direction: the 'masonry-direction' property</h4>
<pre class=propdef>
Name: masonry-direction
Value: row | column | row-reverse | column-reverse
Initial: column
Inherited: no
Applies to: [=masonry containers=]
Computed value: as specified
Animation type: discrete
</pre>
The 'masonry-direction' property
specifies how items are placed
in the [=masonry container=],
by setting its [=masonry axis=] and direction.
<dl dfn-type=value dfn-for=masonry-direction>
: <dfn>column</dfn>
:: The [=masonry container's=] [=masonry axis=] is its [=block axis=],
and masonry layout starts from its [=block-start=] edge.
: <dfn>column-reverse</dfn>
:: The [=masonry container's=] [=masonry axis=] is its [=block axis=],
and masonry layout starts from its [=block-end=] edge.
: <dfn>row</dfn>
:: The [=masonry container's=] [=masonry axis=] is its [=inline axis=],
and masonry layout starts from its [=inline-start=] edge.
: <dfn>row-reverse</dfn>
:: The [=masonry container's=] [=masonry axis=] is its [=inline axis=],
and masonry layout starts from its [=inline-end=] edge.
</dl>
The [=masonry container's=] [=grid axis=]
is perpendicular to its [=masonry axis=].
<h4 id=masonry-fill>
Masonry Cross Direction: the 'masonry-fill' property</h4>
<pre class=propdef>
Name: masonry-fill
Value: normal | reverse
Initial: normal
Inherited: no
Applies to: [=masonry containers=]
Computed value: as specified
Animation type: discrete
</pre>
'masonry-fill' determines what direction auto-placed items are laid out in
when multiple tracks are tied for “shortest”.
<dl dfn-for=masonry-fill dfn-type=value>
: <dfn>normal</dfn>
::
Ties are broken
by filling the startmost track
: <dfn>reverse</dfn>
::
Ties are broken
by filling the endmost track.
</dl>
<h4 id=masonry-flow>
Masonry Flow Directions: the 'masonry-flow' shorthand</h4>
<pre class="propdef shorthand">
Name: masonry-flow
Value: <<'masonry-direction'>> || <<'masonry-fill'>>
</pre>
The 'masonry-flow' property is a [=shorthand=]
for the 'masonry-direction' and 'masonry-fill' properties.
</section>
<h2 id="masonry-track-templates">
Masonry Track Specification</h3>
In the [=grid axis=],
the full power of [=grid layout=] is available for track specification:
* Track sizes, line names, and areas can be specified on the [=masonry container=]’s [=grid axis=],
just like in [=grid layout=].
* The [=explicit grid=] and [=implicit grid=] are formed in the same way as for a regular [=grid container=].
* Items can be [[#masonry-track-placement|placed]] against these grid templates just as in [=grid layout=].
However,
auto-placed items contribute sizing to all tracks,
not just the track into which they are ultimately placed;
see [[#track-sizing]].
Note: This is because auto-placed items must be laid out <em>as</em> they are placed,
so that each track knows how “full” it is
(and therefore which track should receive the next auto-placed item);
thus, the tracks themselves must already have a definite size
so that the items know their [=available space=] during layout.
<section class="option grid">
<h3 id="masonry-track-grid-option">
<span class="option grid">Grid-integrated Option:</span> Declaring Masonry Track Templates: the 'grid-template-*' properties</h3>
The 'grid-template-*' and 'grid-auto-rows'/'grid-auto-columns' properties
(and their shorthands)
apply in the [=grid axis=] of the [=masonry container=]
and establish tracks just as on regular [=grid containers=].
</section>
<section class="option masonry">
<h3 id="masonry-track-masonry-option">
<span class="option masonry">Grid-independent Option:</span> Declaring Masonry Track Templates: the 'masonry-template-*' properties</h3>
Tracks in the [=grid axis=] of a [=masonry container=]
are established with 'masonry-template-tracks',
'masonry-template-areas',
and 'masonry-auto-tracks'.
<pre class=propdef>
Name: masonry-template-tracks
Value: <<'grid-template-columns'>>
Initial: repeat(auto-areas, auto)
Applies to: [=masonry containers=]
Inherited: no
Percentages: refer to correspodning dimension of the content area
Computed value: by computed value type per item in the [=computed track list=]
Animation type: if list lengths match, by [=computed value=] type; otherwise, discrete
</pre>
'masonry-template-tracks' has the same syntax and interpretation as 'grid-template-columns',
but a different initial value.
(Instead of defaulting to no tracks, as in Grid Layout,
it defaults to matching 'masonry-template-areas', if possible,
or otherwise filling the [=masonry container=]
with as many ''grid-template-columns/auto'' tracks as possible.)
<pre class=propdef>
Name: masonry-template-areas
Value: none | <<string>>
Initial: none
Applies to: [=masonry containers=]
Inherited: no
Percentages: n/a
Computed Value: the keyword ''masonry-template-values/none'' or a string
Canonical Order: per grammar
Animation type: discrete
</pre>
'masonry-template-areas' has the same syntax and interpretation as 'grid-template-areas',
except it only takes a single string defining one "row" of area names.
<pre class=propdef>
Name: masonry-auto-tracks
Value: <<'grid-auto-columns'>>
Initial: auto
Applies to: grid containers
Inherited: no
Percentages: refer to correspodning dimension of the content area
Computed value: a [=computed track list=]
Canonical order: per grammar
Animation type: if the list lengths match, by computed value type per item; discrete otherwise
</pre>
'masonry-auto-tracks' has the same syntax and interpretation as 'grid-auto-columns'.
</section>
<h3 id="subgrids">
Subgrids</h3>
[=Subgridding=] allows nested [=masonry containers=] (and [=grid containers=])
to share track sizes.
If the parent's corresponding axis is a [=grid axis=],
the subgridded axis is taken from the parent container
[[css-grid-2#subgrids|as specified for grid containers]];
if the parent's corresponding axis is a [=masonry axis=]...
<section class="option grid">
<span class="option grid">Grid-integrated Syntax:</span>
...the subgridded axis acts like ''masonry''.
Note: If this results in ''grid-template/masonry'' in both axes,
it is resolved as normal for [=masonry containers=] with double-axis ''grid-template/masonry'' templates,
i.e. it acts like ''grid-template-columns: none; grid-template-rows: masonry''.
</section>
<p class="option masonry"><span class="option masonry">Grid-independent Syntax:</span>
...the subgridded axis acts like ''grid-template/none''.
In [=masonry layout=], auto-placed [=subgrids=]
don't inherit any line names from their parent grid,
because that would make the placement of the item
dependent on layout results;
but the subgrid's tracks are still aligned to the parent's tracks as usual.
<div class="example">
Here's a subgrid <a href="examples/subgrid-example-1.html">example</a>:
```css
<style>
.grid {
display: inline-grid;
grid: auto auto 100px / masonry;
align-content: center;
height: 300px;
border: 1px solid;
}
.grid > * {
margin: 5px;
background: silver;
}
.grid > :nth-child(2n) {
background: pink;
}
.grid subgrid {
display: grid;
grid: subgrid / subgrid;
grid-row: 2 / span 2;
grid-gap: 30px;
}
.grid subgrid > * { background: cyan; }
</style>
```
```html
<div class="grid">
<item>1</item>
<item>2</item>
<item>3</item>
<subgrid>
<item style="height:100px">subgrid.1</item>
<item>sub.2</item>
<item>s.3</item>
</subgrid>
<item>4</item>
<item>5</item>
<item style="width: 80px">6</item>
<item>7</item>
</div>
```
<figure>
<img src="images/subgrid-example-1.png">
<figcaption>
The rendering of the subgrid example above.
</figcaption>
</figure>
Note how the subgrid's first item ("subgrid.1") contributes
to the intrinsic size of the 2nd row in the parent grid.
This is possible since the subgrid specified a definite position
so we know which tracks it will occupy.
Note also that trying to subgrid the parent's [=masonry axis=]
results in the subgrid getting [=masonry layout=] in its [=inline axis=].
</div>
A [=subgrid=] that is a [=masonry container=]
can be referred to as a <dfn export>submasonry</dfn>.
<h3 id="repeat-notation">
Track Repetition: the ''repeat()'' notation</h3>
This specification introduces new keywords and masonry-specific behavior
for the ''repeat()'' notation.
<h4 id="repeat-auto-areas">
repeat(auto-areas)</h3>
The new <dfn value for="repeat()">auto-areas</dfn> value for the ''repeat()'' notation
represents the number of repetitions necessary
for the total number of explicit tracks to match
the 'grid-template-areas' / 'masonry-template-areas' value
in effect in the corresponding axis.
If multiple tracks are listed for the repetition,
the final repetition is truncated as necessary
to produce the correct number of tracks.
Note: Unlike ''repeat()/auto-fill''--
which always repeats at least once and always repeats the track listing entirely--
the number of repetitions for ''repeat()/auto-areas'' can be zero
(if there are already enough explicit tracks),
and the final repetition can be partial.
If 'grid-template-areas' / 'masonry-template-areas' is ''grid-template-areas/none'',
this value behaves as ''auto-fill''.
Note: This value applies both to regular [=grid containers=] and to [=masonry containers=].
<h4 id="repeat-auto-fit">
repeat(auto-fit)</h3>
In [=masonry containers=] (as in regular [=grid containers=])
''repeat()/auto-fit()'' acts like ''repeat()/auto-fill'',
but with empty tracks [=collapsed tracks|collapsed=].
However, because placement occurs after track sizing,
[=masonry containers=] use a heuristic
to determine if a track will be occupied:
* All tracks occupied by explicitly placed items are considered occupied.
* With the sum of the spans of all auto-placed items as <var>N</var>,
all unoccupied tracks up to the <var>N</var>th such track
are considered occupied.
All tracks produced by the ''repeat()/auto-fit'' repetition and considered unoccupied by this heuristic
are assumed “empty” and are [=collapsed tracks|collapsed=].
A [=collapsed track=] cannot accept placement of auto-placed items.
Note: It is possible for an auto-placed item to be placed in a track when ''repeat()/auto-fill'' is used
that would be collapsed if ''repeat()/auto-fit'' is used
if there are auto-placed items with a span greater than 1
mixed with explicitly-placed items that leave gaps too small for the auto-placed items.
<h3 id="track-sizing">
Grid Axis Track Sizing</h3>
Track sizing works the same as in [[css-grid-2#algo-track-sizing|CSS Grid]],
except that when considering which items contribute to intrinsic sizes:
* All items explicitly placed in that track contribute, and
* All items with an [=automatic grid position=] contribute
(regardless of whether they are ultimately placed in that track).
<div class="example">
For example, suppose there are two columns in the [=grid axis=]
and that
* Items A, B, and C have no explicit position.
* Item D is explicitly placed into the first column.
In this case, items A, B, C, and D all contribute to sizing the first column,
while only A, B, and C (and not D) contribute to the second column.
</div>
In the case of spanning items with an [=automatic grid position=],
they are assumed to be placed at every possible start position,
and contribute accordingly.
<div class="example">
For example, suppose there are 5 columns in the [=grid axis=],
with the middle having a fixed size of ''100px''
and the other two being ''grid-template/auto''-sized.
For the purpose of track sizing,
an item that spans 2 tracks
and has an intrinsic contribution of 220px
is essentially copied and assumed to exist:
* At grid line 1,
contributing 110px to each of the first two tracks.
* At grid line 2,
contributing 120px to the second track.
* At grid line 3,
contributing 120px to the fourth track.
* At grid line 4,
contributing 110px to the fourth and fifth tracks.
</div>
Note: This algorithm ensures that each track is at least big enough
to accommodate every item that is ultimately placed in it,
and does not create dependency cycles between placement and track sizing.
However, depending on the variation in sizes,
tracks could be larger than necessary:
an exact fit is only guaranteed if
all items are explicitly placed in the [=grid axis=]
or all items are the same size
(or matching multiples of that size, in the case of spanning items).
<h4 id="track-sizing-subgrid">
Subgrid Item Contributions</h4>
When sizing the tracks of either a regular [=grid container=] or a [=masonry container=],
a [=submasonry=] has special handling of items that have an [=automatic grid position=]:
* Any such item is placed into every possible grid track
that could be spanned by the [=submasonry=].
(If the submasonry has a [=definite grid position=], thus only the spanned tracks;
if it has an [=automatic grid position=], then all tracks in the parent grid.)
* Any such item receives the largest margin/border/padding contribution
of each edge at which it could hypothetically be placed.
If the item spans the entire subgrid, it receives both.
(See <a href="https://www.w3.org/TR/css-grid-2/#subgrid-item-contribution">CSS Grid Layout §9</a>.)
<h4 id="track-sizing-performance">
Optimized Track Sizing</h4>
Track sizing can be optimized by aggregating items
that have the same span size and placement
into a single virtual item
as follows:
<ol>
<li>
Separate all the [=masonry items=] into <dfn>item groups</dfn>, according to the following properties:
* the span of the item
* the placement of the item,
i.e. which tracks it is allowed to be placed in
* the item's [=baseline-sharing group=]
Note: For example, an item with span 2 placed in the second track
will be in a different group than an item with span 2 that has an [=automatic grid position=].
<li>
For each [=item group=], synthesize a <dfn>virtual masonry item</dfn>
that has the maximum of every intrinsic size contribution
among the items in that group.
If the items apply [=baseline alignment=],
determine the baselines of the [=virtual masonry item=]
by placing all of its items into a single hypothetical grid track
and finding their shared baseline(s) and shims.
Increase the group's intrinsic size contributions accordingly.
<li>
Place hypothetical copies of each [=virtual masonry item=] into the [=grid axis=] tracks
in every position that the item could potentially occupy,
and run the [[css-grid-2#algo-track-sizing|track sizing algorithm]] with those items.
The resulting track sizes are the [=masonry container's=] track sizes.
</ol>
Note: This optimization should give the same results
as the track sizing description [[#track-sizing|above]];
if not this is an error, please
<a href="https://github.com/w3c/csswg-drafts/issues">report it to the CSSWG</a>.
<h2 id="masonry-track-placement">
Masonry Placement</h3>
In the [=grid axis=],
items can be <em>explicitly placed</em> into tracks and span them using the familiar [=grid-placement properties=]’ syntax.
Auto-placement, however, uses the [[#masonry-layout-algorithm]],
placing each item with an [=automatic grid position=]
into the “shortest” masonry track available.
<div class="example">
Here's a masonry layout <a href="examples/pinterest-with-span.html">example</a>
demonstrating placed and spanning items:
<figure>
<img src="images/example-pinterest-with-span.png">
<figcaption>Rendering of the example above.</figcaption>
</figure>
ISSUE: Need a better example!!!
</div>
<section class="option grid">
<h3 id="masonry-placement-grid-option">
<span class="option grid">Grid-integrated Option:</span> Specifying Masonry Item Placement: the 'grid-column-*' and 'grid-row-*' properties</h3>
The 'grid-column-*' and 'grid-row-*' properties
(and their shorthands)
apply in the [=grid axis=] of the items
and establish placement just as in regular [=grid layout=].
</section>
<section class="option masonry">
<h3 id="masonry-placement-masonry-option">
<span class="option masonry">Grid-independent Option:</span> Specifying Masonry Item Placement: the 'masonry-track-*' properties</h3>
Item placement in the [=grid axis=] of a [=masonry container=]
is established with the
<dfn property>masonry-track-start</dfn> and
<dfn property>masonry-track-end</dfn> properties
(and their <dfn property>masonry-track</dfn> shorthand),
whose syntax and interpretation are analogous to the
'grid-column-start' and 'grid-column-end' properties
(and their 'grid-column' shorthand).
</section>
<h3 id="masonry-slack">
Placement Precision: the 'masonry-slack' property</h3>
<pre class=propdef>
Name: masonry-slack
Value: <<length-percentage>>
Initial: 1em
Percentages: relative to the [=grid-axis=] [=content box=] size of the [=masonry container=]
Inherited: no
Applies to: [=masonry containers=]
Computed value: a computed length
Animation type: discrete
</pre>
[=Masonry containers=] are filled
by placing each [=masonry item=]
in whichever [=masonry track=] is currently the least filled.
When multiple tracks are tied for least-filled,
placing the items in order looks good.
But if tracks are only <em>very slightly</em> different heights,
it can look strange to have them not fill in order,
as the height differences aren't perceived as <em>meaningfully</em> different.
The 'masonry-slack' property specifies what the threshold is
for considering tracks to be “the same height”,
causing them to fill in order.
<dl dfn-type=value dfn-for=masonry-slack>
: <dfn><<length>></dfn>
:: Specifies the <dfn dfn for=masonry>tie threshold</dfn>
for the [=masonry container=].
Placement positions are considered to be equally good (“tied”)
if they are within the specified distance
from the shortest position.
Note: The initial value is a “small” distance (''1em'')
that is probably appropriate to represent “close enough”.
</dl>
Issue: Is ''1em'' the right default?
<h3 id="masonry-layout-algorithm">
Masonry Layout and Placement Algorithm</h3>
For each of the tracks in the [=grid axis=],
keep a <dfn>running position</dfn> initialized to zero.
Maintain also a <dfn>auto-placement cursor</dfn>,
initially pointing to the first line.
For each item in [=order-modified document order=]:
<ol>
<li>
If the item has a [=definite grid position=] in the [=grid axis=],
use that placement.
ISSUE: Should this also update the placement cursor?
Otherwise, resolve its [=grid axis=] placement using these substeps:
<ol>
<li>Starting at the first [=grid axis=] line in the [=implicit grid=],
find the largest [=running position=] of the [=grid axis=] tracks
that the item would span if it were placed at this line,
and call this position <var>max_pos</var>.
<li>Increment the line number and repeat step 2
until the item would no longer fit inside the grid.
<li>Let |possible lines| be the line that resulted in the smallest <var>max_pos</var>,
and all lines that result in a <var>max_pos</var> within the [=tie threshold=] of this <var>max_pos</var>.
<li>Choose the first line in |possible lines| greater than or equal to the [=auto-placement cursor=]
as the item's position in the [=grid axis=];
or if there are none such, choose the first one.
<li>Update the [=auto-placement cursor=] to point to item's last line.
</ol>
<li>
Place the item in its [=grid axis=] tracks
at the maximum of the [=running position=]s
of the tracks it spans.
<li>
Calculate the size of the item's <a href="#containing-block">containing block</a>
and then layout the item.
Set the [=running position=] of the spanned [=grid axis=] tracks
to <code><var>max_pos</var> + [=outer size=] + 'grid-gap'</code>.
</ol>
Note: This algorithm chooses the track
that would result in the item being placed as highly as possible.
If there are ties, it chooses the earliest such track,
<em>after</em> the most recently placed item if possible
(ensuring that it always “moves forward” even in the presence of ties).
If placing items in reverse order (see 'grid-auto-flow'/'masonry-flow'),
the algorithm is analogous but in reverse order.
<h4 id="containing-block">
Containing Block</h2>
The [=containing block=] for a [=grid item=] participating in [=masonry layout=]
is formed by its [=grid area=] in the [=grid axis=]
and the [=grid container=]'s [=content box=] in the [=masonry axis=].
<h4 id="rtl-example">
Placement and Writing Modes</h4>
Note: Like all of [=grid layout=],
masonry layout and placement is sensitive to the [=writing mode=].
For example, for ''direction: rtl'',
items are placed right-to-left rather than left-to-right,
whether the inline axis is a [=grid axis=] or a [=masonry axis=].
<div class="example">
Here's a simple <a href="examples/rtl-grid-axis.html">example</a> using ''direction: rtl'' in the [=grid axis=]:
```css
<style>
.grid {
display: inline-grid;
direction: rtl;
grid: masonry / repeat(4, 2ch);
border: 1px solid;
}
item { background: silver }
item:nth-child(2n+1) {
background: pink;
height: 4em;
}
</style>
```
```html
<div class="grid">
<item>1</item>
<item style="grid-column:span 2">2</item>
<item>3</item>
<item>4</item>
</div>
```
<figure>
<img src="images/rtl-grid-axis.png">
<figcaption>Rendering of the ''direction: rtl'' example above.</figcaption>
</figure>
</div>
<div class="example">
Here's a simple <a href="examples/rtl-masonry-axis.html">example</a>
using ''direction: rtl'' in the [=masonry axis=]:
```css
<style>
.grid {
display: inline-grid;
direction: rtl;
width: 10ch;
column-gap: 1ch;
grid: repeat(4, 2em) / masonry;
border: 1px solid;
}
item { background: silver }
item:nth-child(2n+1) {
background: pink;
width: 4ch;
}
</style>
```
```html
<div class="grid">
<item>1</item>
<item style="grid-row:span 2">2</item>
<item>3</item>
<item>4</item>
</div>
```
<figure>
<img src="images/rtl-masonry-axis.png">
<figcaption>Rendering of the ''direction: rtl'' example above.</figcaption>
</figure>
</div>
<h2 id="intrinsic-sizes">
Sizing Grid Containers</h2>
[[css-grid-2#intrinsic-sizes|Sizing Grid Containers]] works the same as for regular [=grid containers=]
but with the following addendum for the [=masonry axis=]:
The <a>max-content size</a> (<a>min-content size</a>) of a [=grid container=] in the [=masonry axis=]
is the largest distance between the [=grid container's=] [=content-box=] [=start=] edge
and the maximum [=margin-box=] end of all the items,
when sized under a <a>max-content constraint</a> (<a>min-content constraint</a>).
<div class="example">
Here's a simple <a href="examples/grid-intrinsic-sizing-example-1.html">example</a>:
```css
<style>
.grid {
display: inline-grid;
grid: masonry / 50px 100px auto;
grid-gap: 10px;
border: 1px solid;
}
item { background: silver; margin: 5px; }
</style>
```
```html
<div class="grid">