The following tables compare HTML compatibility and support for a number of layout engines. Please see the individual products' articles for further information. Unless otherwise specified in footnotes, comparisons are based on the stable versions without any add-ons, extensions or external programs.
Only HTML 4 and XHTML 1.0 served as text/html (so called "HTML compatible" XHTML) are considered. XHTML 1.0 Strict is necessarily XHTML 1.1 with fewer elements and attributes deprecated. XHTML 2.0 is not considered as it is a redefinition of XHTML, which is not backward compatible. For more XHTML-related comparisons, please refer to comparison of layout engines (XHTML).
Version number, if possible, is provided for feature that is fully supported (based on HTML 4.01). Proprietary extensions are not included.
Values indicate the level of support in the most recent version of the layout engine, or (if a version number is given) in the specified version. Version numbers without any other value indicate the version at which the layout engine first fully supported the feature.
Indicates that the layout engine fully supports this property/element when valid values are used.
Indicates that the property/element is completely ignored.
Indicates that the property/element is understood, but that not all values are supported. Supported values are implemented correctly.
Indicates that the property/element is understood, but that it is not implemented correctly in all cases.
Indicates that the property/element is understood, but supported under an alternate name. May be incomplete or buggy.
Indicates that the property/element is no longer supported.
Elements to present documents in multiple views, which may be independent windows or subwindows. Frame layout is only available in the frameset profiles. All frames are not available in XHTML 1.1 (use generic object elements instead for inline frames).
Elements that specify presentational information. Although not all elements are deprecated, their use is discouraged in favor of stylesheets. Deprecated elements are not available in the strict profiles and XHTML 1.1.
^<q> does not support nested quotes prior to Gecko 1.8.
^For images, a frame will be added to the object. Other content types are not supported and the element is incorrectly implemented as an ActiveX wrapper. Nested objects fallback mechanism is not supported prior to version 7.
^Prior to Internet Explorer 8.0 (trident 4.0), and in earlier compatibility modes, the innerText is submitted instead of the value attribute.