[css-variables] Are custom properties strings?

Is a custom property a token sequence that only serializes as the original string, or is it that original string? The difference can be seen in this example:

.container {
  --x:foo 1.00;
}
.container > div {
  @container style(--x:foo 1) {
    color: green;
  }
}

If custom properties are token sequences, this style query matches, but not if they are strings.

The point of serializing using the original string was to preserve "unknown stuff", and maintaining that idea probably means that we don't transform that unknown stuff before comparisons. Otherwise e.g. style(--uuid: 12345678-12e2-8d9b-a456-426614174000) would match --uuid: 12345678-1200-8d9b-a456-426614174000 (example stolen from https://drafts.csswg.org/css-variables/#serializing-custom-props, then modified).

Note that style(--x:1) would still match e.g. --x:1.00 according to the current plan in https://github.com/w3c/csswg-drafts/issues/8376, since the literal side matches <number>.

@tabatkins

Start with the custom-property serialization discussion in the CSS Variables specification and the style-query example in this issue. Review the linked CSSWG issue 8376 and the cited serialization section, then determine whether comparisons use token sequences or original strings and document the resolved behavior.