HTML

HTML (hypertext markup language) is one of the three pillars of the Web as originally developed by Tim Berners-Lee, along with HTTP and URLs. It is the markup language normally used for Web documents (although many other formats can also be used for material on the Web). It originally was an 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 led 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. One version of this standard has been "frozen" into a W3C HTML5 recommendation as of October 28, 2014, while the ongoing "living standard" is getting regular updates which in due time get frozen into subsequent W3C recommendations such as 5.1, 5.2, etc.

Specs

 * W3C specifications:
 * HTML (1) specification
 * HTML 2.0 specification (see also the RFC)
 * HTML 3.2 specification
 * HTML 4.01 specification
 * HTML 5 specification
 * HTML 5.1 specification (latest)
 * HTML 5.1 2nd Edition 2017-10-03
 * HTML 5.2 specification (latest)
 * HTML 5.2 2017-12-14
 * HTML 5.3 &#91;candidate&#93; specification (latest)
 * HTML specification (latest)
 * XHTML 1.0 specification
 * XHTML 1.1 specification
 * HTML specifications publication status
 * The HTML Landscape enumerates the differences between the W3C HTML 5.0, 5.1 and the WHATWG Living Standard. The source for the landscape site is available here.
 * 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
 * Differences from HTML 4: Living Document
 * Relevant spec links according to the Mozilla Servo project.

HTML vs. XHTML
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., &lt;br>) need an added slash in XHTML to make them self-closing (&lt;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 they 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; its specs say that the underlying HTML document can be expressed in either syntax, and while you're still supposed to pick one or the other, there are very forgiving parsing rules for interpreting the document.

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.

DOCTYPE
HTML and XHTML documents begin with a doctype declaration, which is of a format that had a specific meaning in SGML. Browsers and validators could recognize different doctypes to determine which version of HTML was being used, and browsers sometimes changed between "standards" and "quirks" parsing modes based on the doctype. HTML 5, since it was intended as a "living standard" and was no longer based on SGML, used a mimimalist doctype  designed to trigger standards mode in all browsers, but no longer giving any indication of which specific variety of HTML5 (and up) is in use.

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.

Viewing source
Most browsers have a function to view the source code of the current page. HTML is inherently "open source", though the code might have some degree of obfuscation to make it hard to read in its raw form. This hasn't stopped some people, like a governor of Missouri from claiming "decoding HTML" (viewing the plain text source of it) to be illegal hacking and threatening to prosecute for it.

Viewers

 * Wikipedia's timeline of web browsers
 * The Evolt Browser Archive
 * Opera public downloads

Validators

 * W3C HTML validator
 * Command-line-based validator

Conversions to/from HTML

 * mPDF: convert HTML to PDF
 * Textract: extract text from various document formats including HTML
 * Discussion on converting HTML tables to CSV
 * Pandoc: Document format conversion swiss-army knife

Miscellaneous

 * PBDF: Create documents that are simultaneously valid PDF, HTML, and VirtualBox OVA.

Test suites

 * HTML test suite

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.
 * The Origins of the  Tag
 * CERN's 'The birth of the web'. Includes work on restoring the first website and building a line-mode/terminal web browser simulation.
 * Only 90s Web Developers Remember This
 * List of HTML/XHTML elements past and present
 * History of the Fullscreen API
 * Memoirs From the Browser Wars

Other resources

 * Markdown CSS: makes HTML look like plain text
 * EFF Makes Formal Objection to DRM in HTML5
 * 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"
 * Bug 923590 - Pledge never to implement HTML5 DRM (Bugzilla@Mozilla)
 * Stop standardizing HTML
 * BridgeIt: JavaScript library to add native mobile features to HTML 5 web apps
 * 1 in 10 Americans think HTML is an STD, study finds
 * After 5: The future of HTML
 * What Is HTML5? (free e-book)
 * Towards a Taxonomy of Errors in HTML and CSS
 * Embedding Tabular Metadata in HTML (W3C)