CSS Alert - Google AdSense

[Alien]

First time I've seen this alert in my Google AdSense Account:

Minimize the size of CSS resources on your sites

We have analyzed some of the pages on your sites that serve the most ads, and have detected Page Speed problems that create a highly negative user experience for some users. Frequently viewed pages on your sites are referencing unnecessarily large CSS files that would benefit from comment stripping and whitespace removal. This is called minification. Learn how to fix this problem.