Subgrid

[foolip]

Description
Subgrid makes it possible to nest grids while having the items of the nested grid line up with the main grid. (Simply nesting display: grid doesn't allow this.)

Specification
https://drafts.csswg.org/css-grid-2/

Tests
https://wpt.fyi/results/css/css-grid/subgrid?label=experimental&label=master&aligned

As evident from these tests, only Firefox currently implements Subgrid.

Rationale

• Identified as a pain point in MDN 2020 Browser Compatibllity Report
• Identified as a top compat pain point in State of CSS 2021
• Grid usage in general is high and growing

(Originally proposed by @scottlow in web-platform-tests/rfcs#99 (comment) before this repo was created.)

[foolip]

Based on the State of CSS 2021 results in #7, I think the case for Subgrid is strong.

[gsnedders]

Related to #2 would be the question as to whether we treat subgrid as a separate feature to grid generally, or whether we just extend grid to include subgrid.

[foolip]

I have labeled the tests with the interop-2022-subgrid label here:
https://wpt.fyi/results/css/css-grid/subgrid?label=master&label=experimental&product=chrome&product=firefox&product=safari&aligned&q=label%3Ainterop-2022-subgrid

I excluded two crashtests in the directory, but labeled everything else. If anyone thinks we should include crashtests, please comment.

Is anyone aware of additional test coverage that can/should be added here?

[foolip]

Related to #2 would be the question as to whether we treat subgrid as a separate feature to grid generally, or whether we just extend grid to include subgrid.

I think we should treat it as a separate feature, WDYT?

[karlcow]

What would be the pros and cons on both side of the argument?

If as a separate feature it increases the likelihood of being implemented (because not drowned in grid papercuts), probably separate would be better indeed.

• https://bugs.webkit.org/show_bug.cgi?id=202115
• https://bugs.chromium.org/p/chromium/issues/detail?id=618969
• https://bugzilla.mozilla.org/show_bug.cgi?id=1240834

Everyone is treating it as a kind of a separate feature.

[foolip]

IMHO, main advantage of separating them is that it's easier to understand and communicate. Subgrid is distinct enough to warrant its own entries on caniuse.com and MDN so we'd be in good company.

The main advantage to merging that I can see is that it sets a precedent for significantly updating the test list of a focus area between two years. We might want to do the same for viewport measurement for 2023, and would have sorted out any issues by then.

[foolip]

A proposed list of tests has been labeled with interop-2022-subgrid. It would be great if someone from each browser engine team can look over this list and comment if there are any issues.

[dlibby-]

That list LGTM, but I suspect we'll be adding more tests as part of implementing subgrid. We should probably come up with a process for adding tests to the list or agree that we will only score against the tests in the initial snapshot.