[css-fonts-4] Define `value` of `src` with a valid value definition

[cdoublev]

The value field of src is defined with see prose. Would it be possible to use a valid value definition according the CSS value definition syntax?

I know that src must be parsed in a specific way, similarly to <media-query-list> and <forgiving-selector-list>. So I suggest to set its value with something like <font-src-list> that would reference this specific procedure, which should take advantage of parse a comma-separated list according to a CSS grammar with <font-src> as the CSS grammar.

Also, I think it could be helpfull for CSS authors to remove invalid items from the serialization, like invalid media query or selector. edit: invalid selectors in <forgiving-selector-list> and invalid/unknown media queries in <media-query-list> are now preserved - #8356 (comment), #7595 (comment).

[svgeesus]

Want to make a PR for this?

[cdoublev]

Note that one thing I left undefined is the handling of invalid <font-src>. Should they be preserved or removed? At parse time? At serialization time?

Chrome/FF (at least) currently remove invalid <font-src>, like in <forgiving-selector-list> (at parse time, as specified in step 2):

<style>
  @font-face { src: invalid, local(my-font) }
  :is(;, type) {}
</style>
<script>
  document.styleSheets[0].cssRules[0].cssText; // '@font-face { src: local(my-font); }'
  document.styleSheets[0].cssRules[1].cssText; // ':is(type) { }'
</script>

But people now wants to preserve invalid selectors (see issue linked in my first comment).

In <media-query-list>, it is more complex because of back-compat.

That said, consistency may not be mandatory.

You may want to specify the current interoperable behavior right now, or wait for the outcome of the corresponding issues for <forgiving-selector-list> or <media-query-list>.