The method CSSValidator.new#vendor_extensions_as_warnings! tells the validator to throw warnings insted of errors on vendor extensions.
Throw warnings instead of errors regarding vendor extensions: vendor_…
The repo doesn't contain a gemspec file, so it's hard for us to use our own fork, especially in combination with rspec_w3c_matchers. I would suggest creating a permanent gemspec file for this gem.
Added gemspec file to be able to use our fork with bundler
Fix for enabled Net::HTTP.version_1_1
#16494: POST data to CSS validator (instead of GETting)