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.