The linter found some issues in viewer.html with </input> which isn't required
and a missing closing div in test/resources/reftest-analyzer.html.
The HTML can now be nicely formatted. In order to not break the build for
mozilla-central, the preprocessor has been fixed in order to take into account
the white spaces at the beginning of a comment line.
And finally, make .prettierrc (which is supposed to be either json or yaml)
itself lintable.
