Syntax simplification of CSS <source-size-list>

[cdoublev]

[<source-size-list>#,]? <source-size-value> is nearly equivalent to <source-size-list>#? , <source-size-value>. If you think this is not a breaking change, I propose the second syntax definition.

The current syntax definition may predates comma-elision rules and #?.

Commas specified in the grammar are implicitly omissible in some circumstances [...]:

• all items preceding the comma have been omitted
• [...]

https://drafts.csswg.org/css-values-4/#comb-comma

similarly, the # and ? multipliers may be stacked as #?. These stacks each represent the later multiplier applied to the result of the earlier multiplier.

https://drafts.csswg.org/css-values-4/#component-multipliers

However it can make a difference with grammar-driven parsers, which could produce different data representations:

• list(list(commaList(...sourceSizes), comma), sourceSizeValue) instead of list(commaList(...sourceSizes), comma, sourceSizeValue)
• list(omittedGroup, sourceSizeValue) instead of list(omittedGroup, omittedComma, sourceSizeValue])

The second results (from the proposed syntax) are simpler and more consistent.

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/images.html ( diff )

[zcorpan]

@tabatkins is this equivalent?

[zcorpan]

Note that parsing is defined here https://html.spec.whatwg.org/multipage/images.html#parsing-a-sizes-attribute

The grammar is effectively only for conformance checking.

[cdoublev]

The grammar is effectively only for conformance checking.

Indeed. I created this issue after looking for occurrences of [... , ]? ... in CSS productions but I had forgotten about this algorithm.

I take the opportunity of Tab's review to suggest a simplification (in another PR if needed).

<source-size> = <media-condition>? <length [0,]>

When asked to parse a sizes attribute from an element, parse a comma-separated list according to the CSS grammar, with <source-size> as grammar, and the value of the element's sizes attribute (or the empty string, if the attribute is absent) as input, and let unparsed sizes list be the result.

1. Remove all failure items from unparsed sizes list.
2. If unparsed sizes list is empty, return 100vw.
3. For each unparsed size representing <source-size> in unparsed sizes list
   • If <media-condition> is omitted or evaluates to true, return <length>. If this was not the last item in unparsed sizes list and <media-condition> is omitted, that is a parse error.

[tabatkins]

Yes, the grammar is equivalent. We don't concern ourselves with what internal representation particular parsers might generate for themselves; the two are identical in terms of accept/reject sets.

[zcorpan]

@tabatkins thanks!

@cdoublev I'd rather not change the parsing algorithm, it was carefully written to behave as intended.

[zcorpan]

LGTM but needs a rebase

[cdoublev]

Oups, I forgot to update the head before making this PR, sorry. Done.