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.
Given the limitations of the old pre-processor that's used for CSS/HTML files, this unfortunately isn't as "easy" to implement as it is for JavaScript code.
Since this is the first case where we've wanted to do conditional CSS imports, rather than trying to completely re-write the pre-processor, this patch settles for handling it explicitly in the `expandCssImports` function.
