Missing dfn from CSS-2023

[tabatkins]

The dfn at https://drafts.csswg.org/css-2023/#vendor-prefix defines the terms "vendor-prefix"/"unprefixed"/etc, but this doesn't show up in https://github.com/w3c/webref/blob/main/ed/dfns/css-2023.json. (It looks like every other dfn shows up in the data, just this one is conspicuously missing.)

[tidoust]

The definition gets filtered out because it links to an external spec (CSS2). The code responsible for that in the crawler is at:
https://github.com/w3c/reffy/blob/main/src/browserlib/extract-dfns.mjs#L265-L271

That rule was added to cope with specs that import definitions from other specifications using a form like This specification uses the term <dfn><a href="URL of bar">foo</a></dfn> defined in bar. If the crawler does not filter this out, there will be duplicate definitions of foo in the resulting cross-referencing database, one defined in "bar", and one defined in the spec that imports the definition.

In this particular case, the reference is not really to a dfn in CSS2. I'll try to find a way to relax the rule while still avoiding the creation of duplicate definitions.

[tabatkins]

Ah yes, that's a very good heuristic for the general case. Thanks in advance for fixing it for this specific misfire. ^_^