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File Format
Extension(s) .html, .htm
MIME Type(s) text/html
Released 1990

HTML (hypertext markup language) originally was a SGML based markup language. XHTML is HTML redeveloped using the stricter XML rules. Disagreement over the direction of W3C developments from some of the browser vendors lead to the formation of the Web Hypertext Application Technology Working Group (WHATWG). They maintain the spec for the HTML5 or HTML Next or HTML Living Standard, which is not based on SGML any more. The W3C standardisation group will work to formalise the WHATWG specification as a series of standardised 'snapshots' of the living standard.



  • File extension: .HTML, .HTM
  • MIME type (Internet media type): text/html



In HTML versions prior to HTML 5, there was a "fork" between HTML and XHTML, with the former being SGML-based and the latter XML-based. While the features of both are for the most part very similar, there are some syntactic differences which can trap the unwary, usually not causing any actual problems in rendering in common browsers (which are very forgiving of errors), but preventing validation. For instance, any tags not requiring a matching ending tag (e.g., <BR>) need an added slash in XHTML to make them self-closing (<BR />). This should not be used in HTML. There are some other differences such as HTML tags and attributes being case-insensitive so they can be entered in either uppercase or lowercase, while XHTML is case-sensitive and its standard tags are all lowercase. Some parts of the respective syntaxes won't mix and still validate as either variety, which is a problem when webmasters paste in code from diverse sources (including ad-network and affiliate links and scripts which may have terms-of-service contracts mandating that it be used in an unmodified form). However, HTML 5, which is not directly based on either SGML or XML, is more forgiving of allowing such mixed syntax.

The "forgiving" processing of mixed syntax applies only to documents served with the MIME type "text/html"; if an XML MIME type is used, browsers are supposed to be stricter in interpreting the syntax and rejecting documents which are improper or which are of a form they don't understand.

Nonstandard extensions

The formal specs, of course, do not fully describe the HTML documents in use in the "real world", as quite a number of nonstandard elements, attributes, and other extensions have been implemented in various browsers (including the most popular ones), and also, browsers have tended to be very forgiving of invalid markup, leading to lots of sloppy coding being widespread because "it works in [name of popular browser], so that's all that matters!"

In 2013, the Mozilla organization announced the removal of support for the nonstandard BLINK element, supported in various browsers since being introduced in the 1990s as a Netscape extension, and persisting despite widespread belief that it was annoying. New versions of Firefox and other Gecko-based browsers no longer flash text that is enclosed in this element, as well as in various CSS rules suggesting blinking or flashing.



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