[css-grid-1] Clarify interaction of percentages and intrinsic sizing and the grid algorithm overall #3418

In #1921 (comment) @fantasai wrote:

To clarify, the spec is currently written in conformance with the above resolution (see https://drafts.csswg.org/css-grid-1/#valdef-grid-template-columns-length-percentage and https://drafts.csswg.org/css-grid-1/#intrinsic-sizes). But overall what's happening doesn't seem to be very clearly described.

Afaict, we could describe what is happening as the following (which is then implemented differently in UAs in order to optimize the number of passes):

1. Size the Grid Container (per axis, for both axes)

• If it is definite, just get that size.
• If it is indefinite, run the grid sizing algorithm under min/max-content constraints, treating percentages as auto, in order to find the indefinite size.

2. Size the Grid

• Run the grid sizing algorithm under normal constraints: auto behaves as auto, and percentages resolve against the size of the grid container as calculated in step [css-syntax] Add a Previous Version link to the CR. #1.

Does this seem correct, or are there shortcuts under #1 that aren't reflected here?

In #1921 (comment) @mrego wrote:

Does this seem correct, or are there shortcuts under #1 that aren't reflected here?

Yes, probably that would be fine, despite Blink/WebKit implementation don't do that. So for us it'll imply an extra pass as described in #1 but probably that's just an implementation detail that shouldn't be considered in the spec.

Pulling this more editorial-level work out into this issue so I can close the main one.