Google has been sending out alerts via Webmaster Tools (aka Search Console) notifying people that Googlebot cannot access CSS and JS (javascript) files on their website. In the content of this email it states:
Google systems have recently detected an issue with your homepage that affects how well our algorithms render and index your content. Specifically, Googlebot cannot access your JavaScript and/or CSS files because of restrictions in your robots.txt file. These files help Google understand that your website works properly so blocking access to these assets can result in suboptimal rankings.
They also provide detailed instructions on how to fix this, which is relatively easy. Just modify the robots.txt file to remove the restrictions on CSS and JS files. Then log into Search Console and “Fetch as Google” to validate the fix using the “Mobile: Smartphone” option.
Obviously this is becoming more important in their ranking algorithm, as they are spelling it out plain as day.
So…webmasters…make sure to implement this change to your robots.txt as soon as possible to avoid penalties on Google search.