A flash of unstyled content (or flash of unstyled text, FOUC) is an instance where a web page appears briefly with the browser's default styles prior to loading an external CSS stylesheet, due to the web browser engine rendering the page before all information is retrieved. The page corrects itself as soon as the style rules are loaded and applied; however, the shift may be distracting. Related problems include flash of invisible text and flash of faux text.
The issue was documented in an article named "Flash of Unstyled Content". At first, FOUC appeared to be a browser problem unique to Internet Explorer but later became apparent in other browsers, and has since been described as "a Safari epidemic".
A flash of unstyled content is indifferent to changes in CSS or HTML versions. This problem, which leaves the core content unaffected, originates from a set of priorities programmed into the browser. As the browser collects HTML and all the ancillary files referenced in the markup, the browser builds the Document Object Model on-the-fly. The browser may choose to first display the text so that it can parse the quickest.
Flashes of unstyled content are more prevalent now that HTML pages are more apt to reference multiple style sheets. Web pages often include style references to media other than the browser screen, such as CSS rules for printers and mobile devices. Web pages may import layers of style files, and reference alternative style sheets. Online advertisements and other inserted offsite content, like videos and search engines, often dictate their own style rules within their code block. The cascading nature of CSS rules encourages some browsers to wait until all the style datasets have been collected before applying them.
To emulate a flash of unstyled content, developers can use browser add-ons that are capable of disabling a web page's CSS on the fly. Firebug and Async CSS are such add-ons. Other techniques include manually stopping a page from completing the loading of CSS components. Another option entails using script-blocking tools.
The FOUC problem would normally be a minor occurrence. However with the advent of Google AdSense, FOUC has become a Safari epidemic. Because these Google ads not only execute inline script but access layout information that they often don't even end up using in the page, the problem of FOUC is much more severe than it should be.