Flash of unstyled content
A flash of unstyled content (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 is quite visible and distracting.
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. FOUC behavior has also been documented "as a Safari epidemic..."
FOUC is indifferent to changes in CSS or HTML versions. The problem appears to originate 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 DOM (Document Object Model) on-the-fly. The browser may choose to first display what it can parse the quickest, namely the text.
FOUC is more prevalent, or more frequently observed, 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 alternate 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 data is collected before applying it.
To emulate a FOUC, developers can use browser add-ons that are capable of disabling a web page's CSS on-the-fly. Firebug is one such add-on.
- "Flash of Unstyled Content (FOUC)". Blue Robot. 2001. Retrieved 12 October 2012.
- Dave Hyatt (1 September 2006). "The FOUC Problem". Surfin' Safari. The WebKit Open Source Project. Retrieved 16 October 2012.
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.