Research language agnostic test suites

[seagreen]

If we can find a language agnostic test suite for parsing HTML that could save a lot of work.

I've only spent a few minutes looking into this, but I found some promising leads:

• w3c HTML tests: https://github.com/w3c/web-platform-tests/tree/master/html
• Detailed tests for the html5lib python library, split into a separate repo: https://github.com/html5lib/html5lib-tests