CSS Grid Layout Module Level 3

1. Introduction

This section is not normative.

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.

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 CSS Grid with this new grid item placement strategy and CSS Box Alignment with new alignment features.

1.1. Background and Motivation

Masonry 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 regular multi-column layout, where items are placed vertically in the first column until they must spill 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.

The Pinterest search results page exemplifies this layout:

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.

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.

1.2. Value Definitions

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

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

2. Masonry Layout

Masonry layout is supported for grid containers by specifying the value masonry for one of its axes. This axis is called the masonry axis, and the other axis is called the grid axis.

This allows us to use the full power of CSS Grid in the grid axis. Line names and track sizes can be specified and grid items can be placed into the tracks and span them using grid-column / grid-row as usual. The CSS Box Alignment properties work the same as in a regular grid container in this axis. In the masonry axis however, items are laid out one after another using the § 2.4 Masonry Layout Algorithm.

Here’s a masonry layout example demonstrating placed and spanning items:

Subgrid grid items are supported, but subgridding only occurs in the grid container’s grid axis; see § 8 Subgrids for details.

If masonry is specified for both grid-template-columns and grid-template-rows, then the used value for grid-template-columns is none, and thus the inline axis will be the grid axis.

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.

2.1. Line Name Resolution

Grid item line name resolution works the same as if masonry were replaced with none, i.e. line names are resolved in both axes. The line name resolution works exactly is in CSS Grid.

2.2. Grid Item Placement

Grid items are placed using these steps:

1. Place the items as if the masonry axis had none specified in that axis.
2. For any items that were placed at the first (hypothetical) implicit line in the masonry axis in step 1, those items keep their placement in both axes as their final placement. They will be laid out first in each grid axis track and their grid axis placement is considered definite henceforth. All other items ignore their placement from step 1. Any item with a specified definite placement in the masonry axis that doesn’t result in it being placed at the first implicit line in the masonry axis will be treated as having auto-placement in that axis.
3. Place items using the Masonry layout algorithm below.

(The reason for step 1 above is to determine which items contribute to intrinsic track sizing in the grid axis; see § 2.3 Grid Axis Track Sizing below.)

2.3. Grid Axis Track Sizing

Track sizing works the same as in CSS Grid, except that that the algorithm is run only with the subset of items that have a known grid axis placement. This subset contains the items placed at the first implicit line in the masonry axis in Grid Item Placement above, plus the items that have a specified definite placement in the grid axis, plus the items that span all grid axis tracks (which obviously will be placed at the first implicit line since there is no other way to place them). Other items will be placed in the grid axis by the § 2.4 Masonry Layout Algorithm below and don’t contribute to intrinsic track sizing (because that would create circular dependencies).

2.3.1. repeat(auto-fit)

repeat(auto-fit) behaves as repeat(auto-fill) when the other axis is a masonry axis. The reason for this is that auto-placed items depend on the layout size of their siblings. Removing empty tracks after layout wouldn’t be possible in most cases since it might affect any intrinsic track sizes. Even if all track sizes are definite, the containing block size could change for grid-aligned absolutely-positioned descendants. This makes repeat(auto-fit) impossible to support in a grid container with masonry layout.

2.4. Masonry Layout Algorithm

Items are placed in order-modified document order, but items with a definite placement are placed before items with an indefinite position (as in regular grid layout). For each of the tracks in the grid axis, keep a running position initialized to zero. For each item placed at the first implicit line in the masonry axis in Grid Item Placement step 1 above:

1. Position the item at the content edge of the grid container in the masonry axis and at its start track in the grid axis.
2. Calculate the size of the containing block and then layout the item. Then, calculate its resulting margin-box in the masonry axis. Set the running position of the grid axis tracks the item spans to the maximum of margin-box-end + grid-gap and the current running position of those tracks.

Note: This means that items with a definite placement at line 1 in the masonry axis by Grid Item Placement step 1 can be made to intentionally overlap.

The remaining items get their final placement using the following steps:

1. If the item has an indefinite placement in the grid axis, then resolve its definite placement in the grid axis using these substeps:
   1. Starting at the first grid axis line in the implicit grid...
   2. 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 max_pos.
   3. Increment the line number and repeat step 2 until the item would no longer fit inside the grid.
   4. Pick the line that resulted in the smallest max_pos as the item’s definite placement in the grid axis.
   Otherwise, a definite placement in the grid axis is honored the same as in a regular grid.
2. Position the item at its grid axis start track and the maximum of the running positions of the tracks it spans.
3. Calculate the size of the item’s containing block and then layout the item. Then calculate its resulting margin-box in the masonry axis. Set the running position of the grid axis tracks the item spans to max_pos + margin-box-end + grid-gap.

2.5. The masonry-auto-flow Property

The § 2.4 Masonry Layout Algorithm above can be modified in two ways, using the new masonry-auto-flow property:

First, picking definite items first for placement can be inhibited by specifying the ordered keyword so that a plain order-modified document order is used instead. Second, instead of placing the items in the track(s) with the most remaining space as described above we can place them one after another in the grid axis by specifying the next keyword, for example:

<style>
.grid {
  display: inline-grid;
  grid: masonry / repeat(3, 2ch);
  border: 1px solid;
  masonry-auto-flow: next;
}

item { background: silver }
item:nth-child(2n+1) {
  background: pink;
  height: 4em;
}
</style>

<div class="grid">
  <item>1</item>
  <item>2</item>
  <item>3</item>
  <item>4</item>
</div>

(Without masonry-auto-flow: next, item 4 would be placed below item 2.)

3. Containing Block

The containing block for a grid item is formed by its grid area in the grid axis and the grid container’s content-box in the masonry axis.

4. The Implicit Grid

Here’s a simple example using direction: rtl in the grid axis:

<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>

<div class="grid">
  <item>1</item>
  <item style="grid-column:span 2">2</item>
  <item>3</item>
  <item>4</item>
</div>

Here’s a simple example using direction: rtl in the masonry axis:

<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>

<div class="grid">
  <item>1</item>
  <item style="grid-row:span 2">2</item>
  <item>3</item>
  <item>4</item>
</div>

5. Sizing Grid Containers

Sizing Grid Containers works the same as for regular grid containers but with the following addendum for the masonry axis: The max-content size (min-content size) 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 max-content constraint (min-content constraint).

Here’s a simple example:

<style>
.grid {
  display: inline-grid;
  grid: masonry / 50px 100px auto;
  grid-gap: 10px;
  border: 1px solid;
}
item { background: silver; margin: 5px; }
</style>

<div class="grid">
  <item style="border:10px solid">1</item>
  <item>2</item>
  <item>3</item>
  <item style="height:50px">4</item>
  <item>5</item>
  <item>6</item>
</div>

6. Alignment and Spacing

Gutters are supported in both axes. In the masonry axis, the gap is applied between the margin boxes of each pair of adjacent items. Margins do not collapse in either axis.

In the grid axis, alignment works the same as in a regular grid container.

In the masonry axis, content-distribution is applied to the content as a whole, similarly to how it behaves in block containers. More specifically, the alignment subject is the masonry box, which is the area between the content-box edge of the grid container and the margin-box end edge of the item that is the furthest away in the masonry axis, as indicated by the dashed border here:

Note that there is only ever one alignment subject for these properties in the masonry axis, so the unique align-content / justify-content values boil down to start, center, end, stretch and baseline alignment. (normal behaves as stretch as usual for grid containers). In the figure above, the grid container has align-content: start; but if align-content were at its default normal value, then the masonry box would fill the grid container's content box, due to being stretched. Moreover: if the grid items overflowed the grid container's content box in the masonry axis, then the masonry box would be larger than the grid container's content box.

6.1. The align-tracks and justify-tracks Properties

This specification adds two new properties to align the items in the masonry axis for each individual grid axis track:

Note: These values are the same as for align-content / justify-content, but here we accept multiple values in a comma-separated list.

Unlike align-content / justify-content, normal behaves as start for these properties. So the default rendering is the expected packed masonry layout as shown in the top left corner in the example below. When multiple values are specified, the first track in the grid axis uses the first value, the second track uses the second value, etc. If there are fewer values than tracks, then the last value is used for the remaining tracks. If there are more values than tracks, then the remaining values have no effect on the rendering.

Here’s a testcase that demonstrates a few align-tracks alignment possibilities.

6.2. Stretch Alignment in the Masonry Axis

stretch will stretch the items in the masonry axis for each track separately, to fill the grid container’s content-box. Any item can opt out from stretching by setting align-self / justify-self to something other than normal or stretch in the relevant axis. A stretched item with an indefinite size in the masonry axis will stretch its content-box size, but only up to its max size. An item with a definite size and an auto margin in the masonry axis will stretch by increasing auto margin(s). Otherwise, the item doesn’t stretch.

6.3. Individual Track Alignment in the Masonry Axis

align-tracks / justify-tracks values can be specified per track.

Here’s an example to illustrate this. Note that this example’s align-tracks value intentionally has one value less than the number of tracks to illustrate that the remaining track(s) use the last value (i.e. the right-most track also uses space-evenly). (baseline values are also supported but excluded in this test, see § 6.4 Baseline Alignment in the Masonry Axis for a description how that works with examples.)

6.4. Baseline Alignment in the Masonry Axis

Item baseline alignment inside the grid axis tracks works as usual for a regular grid container, and the grid container’s baseline is determined the same as for a regular grid container in that axis.

Baseline alignment is supported also in the masonry axis, on the first and last item in each track (but not on items "in the middle" of the track). Only tracks with the align-tracks / justify-tracks values start, end or stretch support baseline alignment. There are four different sets of baseline-sharing groups:

1. the first item in each start track + the first item in each stretch track
2. the last item in each start track
3. the first item in each end track
4. the last item in each end track + the last item in each stretch track

Each of those sets can have a first baseline set and a last baseline set, resulting in eight unique baseline sets in the masonry axis.

specify how the grid container’s first(last) baseline in the masonry axis is determined

Here’s an example illustrating all eight possibilities. (The example uses two separate grid containers to illustrate the first ("F") and last ("L") baseline sets to avoid cluttering the illustration, but it is possible to use all eight sets in the same container.) The aligned baselines are indicated with red color. Note that the tracks that are attached to the end side of the masonry box adjust the padding (or margin in the case of align-self) on the end side, whereas tracks attached to the start side adjust the start padding/margin. (only align-content is used in this example, which adjusts the padding, since it’s easier to see the baseline adjustment. align-self can also be used, or a mix of the two, as usual.)

this needs more details about edge cases, caveat about misalignment in stretch, etc

Authors are advised to be careful with using alignment values other than start when some items span more than one track, because it’s easy to cause items to unintentionally overlap in this case.

maybe we can make stretch alignment (at least) smarter so that we avoid overlapping spanning items in a few more cases that would be useful to authors?

7. Fragmentation

7.1. Fragmentation in the Masonry Axis

Each grid axis track is fragmented independently in the masonry axis. If a grid item is fragmented, or has a forced break before/after it, then the running position for the tracks that it spans in the grid axis are set to the size of the fragmentainer so that no further items will be placed in those tracks. An item that is split into multiple fragments retains its placement in the grid axis for all its fragments. A grid item that is pushed, however, is placed again by the next grid container fragment. Placement continues until all items are placed or pushed to a new fragment.

7.2. Fragmentation in the Grid Axis

Fragmentation in the grid axis with masonry layout in the other axis is also supported. In this case the fragmentation behaves more like in a regular grid container; however, there’s a separate step to determine which grid-axis track each item is placed into, before fragmentation occurs.

8. Subgrids

Masonry layout is supported in subgrids (e.g. grid: subgrid / masonry), and grids that use masonry can have subgrids as children. However, only a parent grid axis can be subgridded in the normal sense. A subgrid axis with a parent masonry axis will behave as masonry, unless the subgrid’s other axis is also masonry in which case it behaves as none (because a grid container can only have one masonry axis). auto-placed subgrids don’t inherit any line names from their parent grid, because that would make the placement of the subgrid’s grid items dependent on layout results; but the subgrid’s tracks are still aligned to the parent’s tracks as usual. Here’s a subgrid example:

<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>

<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>

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 placement 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.

9. Absolute Positioning

Grid-aligned absolute-positioned descendants are supported. In the masonry axis, all grid positions except line 1 are treated as auto. Line 1 in the masonry axis corresponds to the start of the masonry box (which is usually also the content-box start edge) and auto uses the grid container padding edge as usual. The containing block is the extent of the tracks the item spans in the grid axis and the position of line 1 and the padding-box edge in the masonry axis.

It might be useful to define a static position in the masonry axis. Maybe it could defined as the max (or min?) current running position of the grid-axis tracks at that point? Or the end of the item before it?

It would also be useful to be able to align the masonry box end edge somehow, but for that we need a way to address the end line in an implicit grid, or could we just use any non-auto line number other than 1 to indicate the end line given that we don’t really have any lines in this axis other than line 1?

10. Performance Notes

In general, masonry layout should have significantly better performance than the equivalent regular (2-axis) grid layout, particularly when the masonry axis is the block-axis since the intrinsic sizing of grid rows is typically quite expensive. Any intrinsic track sizing in the grid axis should be cheaper too, because, typically, only a subset of items contribute to the intrinsic sizing in a masonry layout, contrary to a 2-axis grid where all items spanning an intrinsically-sized track contribute. That said, align-tracks: stretch specifically adds a cost proportionate to the number of items that are resized. (Note that stretch isn’t the default value for these properties though.) Stretched items do a second layout with the new size (when it actually changed) so this can be costly if there are a huge amount of stretched items that each contains a lot of content. Especially nested stretched masonry layouts should be avoided unless they are small/trivial.

This can be ameliorated by the author by opting out from the stretching on most items though, e.g. specifying align-items:start and then opting in for just a few items with align-self:stretch to let those items fill the masonry axis. Other justify-tracks / align-tracks values such as center, end and '<content-distribution>' (other than stretch) shouldn’t be a problem though since they just reposition the items which is fast. (This performance analysis is from a Gecko perspective, but I suspect there’s some truth to it for other layout engines as well.)

11. Graceful Degradation

Typically, a masonry design can be expected to degrade quite nicely in a UA that supports Grid layout but not masonry layout if the grid/grid-template shorthands are avoided and the longhands are used instead. e.g.

grid-template-rows: masonry; /* ignored by UAs that don’t support it */
grid-template-columns: 150px 100px 50px;

12. Acknowledgements

Thanks goes to Cameron McCormack who wrote a masonry layout explainer document (from which I lifted the Background chapter) and presented it to the CSSWG. Thanks also to everyone who provided feedback on the initial proposal for this feature.