[css-values-4] Allow src() to take multiple arguments

[LeaVerou]

In #541 we resolved to add src() as a url() alias without the weird parsing rules around unquoted strings that prevent url() from containing var() references, and @tabatkins made the edits.

However, for var() to truly be useful with URLs and cover the main use cases, there are two more pieces of the puzzle: concatenation and casting to string. There is #542 where we discuss proposals for syntax to do both, but so far they have not reached enough consensus (although there is mostly consensus on the necessity).

I was thinking, since the main use case for concatenating strings and casting to string is building dynamic URLs (especially SVG data URIs), what if we just allow src() to take multiple arguments, and concatenate (and cast, as needed) the result?

E.g. something like this would then work:

select {
	background-image: src('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 50">\
		<path d="m 20 10 L 50 40 L 80 10" fill="none" stroke="', currentColor, '" stroke-opacity=".4" stroke-width="13" stroke-linejoin="round" stroke-linecap="round" />\
	</svg>');
}

Which would address a lot of today's hacks wrt icons in one fell swoop.

[fantasai]

One of the main reasons to have src() was also to allow it to take multiple arguments, see e.g. #1603 so whatever syntax you want to introduce for string concatenation needs to be compatible with that as well.

Overall, I think I'd rather introduce syntax for general concatenation, even if we only allow it within src() for now, rather than do something specific to src() that complicates our ability to expand its syntax later.

[tabatkins]

Yeah, given that one of the "casting" use-cases is keyword->string, building this directly into src() seems a little fraught with the other fetch-controlling keywords we've been planning to add to url()/src() forever.

Speccing out string() or whatever wouldn't be any more effort than speccing the exact same behavior in src(), so let's just do that.

[LeaVerou]

Fair enough. And it would be strictly more powerful too. I'm gonna go ahead and close this then.