Lint or validate HTML fixtures in CI

[Weibye]

What problem does this solve or what need does it fill?

Our test fixtures are humanly typed HTML.

• There has been some cases of typos in the html which produces invalid tests. Update wrong css in test #352
• There was also a case of a missing semicolon somewhere, but I can't seem to find the PR or commit.

What solution would you like?

• Some form of lint or validation in CI to ensure that the HTML is correct and without obvious flaws. This would help verify that our test results are correct and can be trusted (more).

What alternative(s) have you considered?

Do nothing, it's not a large cause of issues at the moment.