Add HTML validation
!2175 (merged) removed HTML validation.
We didn't find an easy drop-in way to validate HTML 5.
One option is to use Nu Html Checker (https://github.com/validator/validator/)
Nu Html Checker provide a command line tooling in Java - it would work but it seems slow to load (around 2 seconds in a slow VPS).
The Nu Html Checker also offers a way to run it as a service that is likely to be fast enough (but we would need to integrate it):
e.g. launch with:
bin/java nu.validator.servlet.Main 8888
And use it:
bin/java nu.validator.client.HttpClient FILES
In that slow VPS it goes from around 1.8 - 2.0 seconds to 0.2 seconds. And it's likely that there is a JSON interface so we could avoid launching a new Java process for validating each file.