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Technique used by web browsers to maintain backwards compatibility with older web pages

In computing, quirks mode is an approach used by web browsers to maintain backward compatibility with web pages designed for old web browsers, instead of strictly complying with web standards in standards mode. This behavior has since been codified, so what was previously standards mode is now referred to as simply no quirks mode.

One prominent difference between quirks and no quirks modes is the handling of the CSS Internet Explorer box model bug. Before version 6, Internet Explorer used an algorithm for determining the width of an element's box which conflicted with the algorithm detailed in the CSS specification, and due to Internet Explorer's popularity many pages were created which relied upon this non standard algorithm. As of version 6, Internet Explorer uses the CSS specification's algorithm when rendering in no quirks mode and uses the previous, non standard algorithm when rendering in quirks mode. Another notable difference is the vertical alignment of certain types of inline content; many older browsers aligned images to the bottom border of their containing box, although the CSS specification requires that they be aligned to the baseline of the text within the box. In no quirks mode, Gecko based browsers will align to the baseline, and in quirks mode they will align to the bottom. Additionally, many older browsers did not implement inheritance of font styles within tables; as a result, font styles had to be specified once for the document as a whole, and again for the table, even though the CSS specification requires that font styling be inherited into the table. If the font sizes are specified using relative units, a standards compliant browser would inherit the base font size, then apply the relative font size within the table: for example, a page which declared a base font size of 80% and a table font size of 80% (to ensure a size of 80% in browsers which do not properly inherit font sizes) would, in a standards compliant browser, display tables with a font size of 64% (80% of 80%). As a result, browsers typically do not inherit font sizes into tables in quirks mode.

A third compatibility mode known as "limited quirks mode" (previously, "almost standards mode" or "strict mode"), which maintains the "traditional" vertical sizing of table cells according to the CSS2 specification, has been implemented in these browsers: Safari, Opera 7.5 (and later), all Gecko based browsers since 1.0.1 (such as Firefox) and Internet Explorer 8.

Henri Sivonen compiled a list of various document types and how they are treated in the most common browsers, showing whether pages are rendered in Quirks, Standards, or Almost standards mode. The criterion used for "Almost standards mode" is non standard table cell height rendering. This table applies to content served with the Content Type text/html. Content served with the Content Type application/xhtml+xml is rendered in Standards mode in Chrome, Firefox, Internet Explorer 9, Safari, and Opera. Internet Explorer 6, 7, and 8 do not support Content Type application/xhtml+xml. Additionally, in Mozilla Firefox and Opera the rendering mode in use for a given page is indicated on the 'Page info' informational box.