Hooray! Great to have this.

I made some changes. To summarize:

• I encapsulated the code a bit more, reducing the number of local variables in the already quite large and stateful build function. It would be nice to have separate unit tests for validateLinks, perhaps, but it’s pretty simple so not urgent.
• I moved the broken link warning to the end where it’s more likely to be seen. If we want to make this a hard error, then we can move it earlier.
• I renamed links to localLinks to indicate that we’re only extracting (and validating) local links, not all links.
• I added encoding as appropriate (${path}#${encodeURIComponent(id)}).
• I skipped validation of <a href download> since these links are rewritten to point to files.
• I disallowed (non-download) links to files, since these links are not rewritten and would be broken.
• I would like to allow links to exported files, but I was lazy and didn’t implement this yet.
• I allowed links to exported modules (which is unlikely but should work).
• I added normalization for trailing .html (in addition to the existing /index).
• I used parseRelativeUrl instead of hacking URL with proto:….
• I fixed a longstanding quirk in resolvePath (and added tests).
• I added unit tests for the new findAssets logic.