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.

The structure and appearance of a web page is described by a combination of two standardized languages:

However, most older web browsers either did not fully implement the specifications for these languages or were developed prior to the finalization of the specifications (Microsoft Internet Explorer version 5.0 for the Macintosh platform, released in 2000, was the first major web browser with full support for CSS Level 1, for example). As a result, many older web pages were constructed to rely upon the older browsers' incomplete or incorrect implementations, and will only render as intended when handled by such a browser.

Support for standardized HTML and CSS in major web browsers has increased significantly, but the large body of legacy documents which rely on the quirks of older browsers represents an obstacle for browser developers, who wish to improve their support for standardized HTML and CSS, but also wish to maintain backward compatibility with older, non-standardized pages. Additionally, many new web pages continue to be created in the older fashion, since the compatibility workarounds introduced by browser developers mean that an understanding of standardized methods is not strictly necessary.

To maintain compatibility with the greatest possible number of web pages, modern web browsers are generally developed with multiple rendering modes: in "no quirks mode" pages are rendered according to the HTML and CSS specifications, while in "quirks mode" attempts are made to emulate the behavior of older browsers. Some browsers (those based on Mozilla's Gecko rendering engine, or Internet Explorer 8 in strict mode, for example) also use an "almost standards" mode which attempts to compromise between the two, implementing one quirk for table cell sizing while otherwise conforming to the specifications.

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. ^{[citation needed]}

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.