The HTML page returned by GitHub no longer contains ID attributes for
line numbers or headers (from user's Markdown files); the functionality
is implemented in JavaScript now.
Among other changes, this patch also disables HTML validation. This is a
bit tricky to get right, because when the Markdown is processed by
GitHub, it does some transformations that aren't mimicked by Pandoc. The
result of converting the existing Markdown to HTML using Pandoc is in
fact invalid HTML5.
The best way to fix this would be to use the [GitHub Markdown
API](https://docs.github.com/en/rest/markdown) to render the Markdown,
but that doesn't seem worth the effort right now. Instead, this patch
just disables HTML validation but preserves the link checking
functionality (and makes it start passing again).
For some reason, GitHub is returning a 403 to the GitHub actions runner
when it tries to access these URLs. Maybe docs.github.com doesn't allow
access from GitHub actions.
Andrew Oh