Make HTML's html-dfn.js work

[annevk]

Corresponding HTML PR: whatwg/html#12523.

[annevk]

@tobie let me know if this seems reasonable or if you'd approach this differently. (I used Claude to make this.)

[tobie]

Thanks for the PR. Could you explain what the intent is? I'm not sure I entirely understand the problem this is trying to solve. Admittedly, I haven't looked at that code base for a while.

[annevk]

Oh oops. The HTML standard uses html-dfn.js to make back-references work. I.e., going from a dfn elements to all the places it is referenced. html-dfn.js relies on xrefs.json for that which is generated by Wattsi.

Then:

1. Currently PR Preview does not copy over these files.
2. The HTML standard assumes they are at / which does not work for PR Preview.

If we only fixed 2 we would not get back-references for newly-added definitions, which is why I'm attempting a fix here.

[tobie]

What is the rationale for rewriting the file paths rather than making them relative?

[annevk]

We use this for both single-page and multi-page. There's not a relative path that works for both. That's why I kept the logic in PR Preview. An alternative would be having PR Preview invoke Wattsi in a specific way perhaps and make that influence the output somehow. This seemed the most straightforward.

[tobie]

This feels brittle and hacky, but as you’re the only consumer of the Wattsi codenpath, if you’re fine with it, I don’t have any objections.