User blog comment:Ducksoup/Visual Editor Gets an Update/@comment-13918697-20150329000235/@comment-452-20150330212822

It was announced that it was a feature they were developing, it was not announced that it would be rolled out so soon. You can always turn the highlighting off in your preferences if you don't like it. :) Unless you don't like it on CSS and JS pages, in which case you're stuck with it.