A CSS hack is a coding technique used to hide or show CSSmarkup depending on the browser, version number, or capabilities. Browsers have different interpretations of CSS behavior and different levels of support for the
W3C
The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working to ...
standards. CSS hacks are sometimes used to achieve consistent layout appearance in multiple browsers that do not have compatible rendering. Most of these hacks do not work in modern versions of the browsers, and other techniques, such as feature support detection, have become more prevalent.
Types of hacks
Invalid or non-compliant CSS
Due to quirks in the interpretation of CSS by various browsers, most CSS hacks involve writing invalid CSS rules that are interpreted only by specific browsers, or relying on bugs in specific browsers. An example of this is prefixing rules with an underscore (as in _width) to target Internet Explorer 6—other browsers will ignore the line, allowing it to be used to write code specific to one browser.
Similar CSS hacks involve inducing syntax errors like asterisks, missing whitespace, and CSS comments around property names. Additionally, in
Internet Explorer
Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft which was used in the Windows line of operating systems ( ...
6 and 7, the !important declaration is recognized as such with any string after the exclamation mark, e.g. !ie.
Unsupported CSS
Although newer CSS rules are correct by current standards, they are ignored by older browsers as "invalid". By writing old rules followed by newer rules that cancel out or modify the old ones, it is possible to only activate certain rules on older browsers.
Conditional comments
Prior to version 10,
Internet Explorer
Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft which was used in the Windows line of operating systems ( ...
supported a special comment syntax that would allow blocks of HTML to be read only by specific versions of the browser. These comments are mostly used to provide specific CSS and JavaScript workarounds to older versions of the browser. No other browsers interpreted these comments or offered similar functionality.
The following are examples of the different syntax for these comments.
Test
GNORE[>__
____