Every now and then I encounter a problem where several sites that I visit fail to displays CSS. It looks something like this (Gizmodo website):
The problem appears when something goes wrong while you are deleting browser cache,cookies etc. In my case it usually happens when I use CCleaner (free tool for cleaning your Windows PC) and I accidentally open Firefox (or other browser) while CCleaner is deleting the cache. Websites start to look like above screenshot and refreshing the page doesn’t help.
The solution is to refresh the page with CTRL + F5, which forces browser to redownload the entire site from internet and not from local cache. This solves problem only for that site so what I do is close all browsers and run CCleaner again.