A flash of unstyled content 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.
Technical information
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". FOUC is indifferent to changes in CSS or HTML versions. The problem 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, which it can parse the quickest. FOUC is 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 data is collected before applying it. With the advent of JavaScript libraries such as jQuery that can be employed to further define and apply the styling of a web page, FOUC has also become more prominent. In an attempt to avoid unstyled content, front-end developers may choose to hide all content until it is fully loaded, at which point a load event handler is triggered and the content appears. To emulate a FOUC, 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. While, by 2016, several different techniques had been developed to avoid undesired display behaviours, a change in rendering behaviour in Google Chrome version 50, whereby stylesheets injected by JavaScript are prevented from blocking page loading, as required by the HTML5 specification, brought the situation to website creators' attentions again, particularly affecting users of Typekit, a web typography product from Adobe Systems. Within 2 months, Adobe had changed the way in which their fonts were included into third-party websites in order to avoid the undesired rendering behaviour.
