#foo { clip: rect(0 250px 20px 0); clip: rect(0,250px,20px,0); }
CSS validator says:
"Value Error : clip Invalid separator in shape definition. It must be a comma. : rect(0 250px 20px 0)"
The word "clip" is a link to the CSS 2.0 definition of clip, which due to a mistake, says it should be a space, not a comma. This was fixed in CSS 2.1, but validator links to the buggy version (amongst other things, it also says that clip only applies to block-level and replaced elements with non-visible overflow, none of which are corrected in the errata).

Since the idea of making the style name into a link is to let the author see the correct syntax, it would be better to link to the specification that does not have the mistake.

CSS 2.1 says:
"User agents must support separation with commas, but may also support separation without commas, because a previous revision of this specification was ambiguous in this respect."