[css-grid-2] option to specify an optional sequence of track sizes

Spec: https://www.w3.org/TR/css-grid-1/#auto-repeat

In a number of scenarios flexbox and grid are able to respond to their content and container to achieve layouts that people want container queries for (which seem unlikely to happen). Being able to define a completely collapsible sequence of tracks would allow things like switching from a layout of X columns * 1 row to 1 column wide * X rows depending on the size of the container. In my case I'm looking at ways to switch between a 3×1 and 1×3. This would also allow things like odd numbers of columns/rows down to 1 instead of 3.

This could work with repeat so it could be used with auto-fill/fit, maybe as an additional keyword - repeat(3 optional, ...) or repeat(auto-fit optional, ...)?

Start with the linked CSS Grid auto-repeat specification section and compare its existing repeat(), auto-fill, and auto-fit behavior with the requested collapsible track sequence. Clarify the intended syntax and layout behavior for cases such as 3×1 changing to 1×3; completion would require a CSSWG decision and corresponding specification update.