Discover HTML element attributes via webref?

[Elchi3]

Currently, we have to add HTML attributes manually and so we don't always discover new attributes. See https://github.com/openwebdocs/mdn-bcd-collector/blob/main/custom/elements.json

Can we improve https://github.com/openwebdocs/mdn-bcd-collector/blob/main/test-builder/elements.ts so that more attributes are discovered via webref?

See also #3059