What happens in Quirks Mode?

Quirks Mode is a mode of operation of web browsers such as Internet Explorer (IE), Firefox, and Opera. Basically, Quirks Mode (also called Compatibility Mode) means that a relatively modern browser in ten tio nal ly simulates many bugs in older browsers, especially IE 4 and IE 5.


Quirks Mode is a mode of operation of web browsers such as Internet Explorer (IE), Firefox, and Opera. Basically, Quirks Mode (also called Compatibility Mode) means that a relatively modern browser in ten tio nal ly simulates many bugs in older browsers, especially IE 4 and IE 5.

Quirks Mode is triggered by doctype sniffing, also known as doctype switching. This means that the browser inspects the start of an HTML document to see whether it contains a doctype declaration as required by HTML specifications.
The purpose of Quirks Mode is to make old pages to display as their author intended. Old pages may have been written to utilize known features of old browsers or at least to adapt to them. For more information about Quirks Mode in general, consult the QuirksMode.org site.

There is no authoritative specification of what happens in Quirks Mode. After all, the mode is, by essence, an inten tional violation of CSS and HTML specifications. However, since authors may need a description of what may actually happen in Quirks Mode, I have composed this document.

If you have an existing page that seems to work well but lacks a doctype dec la ra tion (required by HTML specifications) at the beginning, you should not simply put such a dec la ra tion there. The reason is that the dec la ra tion makes browsers work in the so-called Standards Mode as opposite to Quirks Mode. This may mean just about anything. I have seen the entire content of a page disappear when you add a doctype dec la ra tion. More often, the layout changes in rather un ex pect ed ways. Well, the ways are not that un ex pect ed if you know what may happen in Quirks Mode. Before adding a doctype dec la­ra tion, you should check both the HTML and CSS code for syntactic correctness by using validators and checkers. This may not be enough, since the page might still have been written relying on things that just “work”—in Quirks Mode. Thus, you should test the page at least on IE 7 and Firefox 2 in Standards Mode, i.e. after adding a doctype dec la ra tion. If the page doesn’t work then, the following list might be useful for spotting the problem.
When creating a a new page, you need not know about Quirks Mode and should usually not think about it. Simply write according to HTML and CSS specifications; this includes using a doctype dec la ra tion that puts modern browsers into Standards Mode. Moreover, put the doctype declaration at the very start, since some browsers go to Quirks Mode, if there is anything (even a comment) before it. (This causes trouble if you are using XHTML, but in most cases, you shouldn’t use XHTML for web pages anyway, for the time being.) But if you decide to use some features that might only work in Quirks Mode, such as a height="100%" attribute for a table element, you should check the list for other possible implications.