What is Flash of Unstyled Content or FOUC
Some pages that use the CSS @import rule experience a curious display quirk in Chrome, Internet Explorer and FireFox (Sometimes). It usually happens when we use optimization plugins like Above the Fold Optimization (in my case).
How do I prevent Flash of Unstyled Content?
Well Unstyled CSS Flash is not something that hurt your website or visitors, but still if you want to prevent FOUC. Then stop using CSS optimization plugins.
If you need more help, I recommend you to read this article on TechRepublic.com