HTML

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.

Identifiers

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

Specs

 * W3C specifications:
 * HTML (1) specification
 * HTML 2.0 specification (see also the RFC)
 * HTML 3.2 specification
 * HTML 4.01 specification
 * HTML5 working draft
 * XHTML 1.0 specification
 * XHTML 1.1 specification
 * Web Hypertext Application Technology Working Group (WHATWG) specifications:
 * The WHATWG Wiki
 * HTML Living Standard
 * Blog post describing the background information of the living standard
 * List of all WHATWG specifications
 * List of 'HTML derivatives' and other spin-off specifications
 * HTML Snapshots

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.

Software

 * mPDF: convert HTML to PDF

Validators

 * W3C HTML validator

Test suites

 * HTML test suite

Format conversion

 * Discussion on converting HTML tables to CSV

Historical information

 * W3C Web History Community Group
 * Tim Berners-Lee discusses Web protocols/formats in Jan 1992
 * Dive into HTML5 - How did we get here? also documents how HTML has developed.
 * The Lost Tags of HTML, documenting early HTML versions and the tags that have been dropped from the standards.

Other resources
^ W3C green-lights adding DRM to the Web's standards, says it's OK for your browser to say "I can't let you do that, Dave"
 * Markdown CSS: makes HTML look like plain text
 * EFF Makes Formal Objection to DRM in HTML5