Formerly just a recommendation, it is now officially a Google webmaster guideline. It basically says to not block page resources like CSS and JavaScript….
“To help Google fully understand your site’s contents, allow all of your site’s assets, such as CSS and JavaScript files, to be crawled.”

A Google guideline check tool will display any page resources that are blocked by your robots.txt file. To make sure you are following this new guideline, use a third-party guideline check tool, like or similar.