In computing, HyperText Markup Language (HTML) is a markup language designed for the creation of web pages with hypertext and other information to be displayed in a web browser. HTML is used to structure information — denoting certain text as headings, paragraphs, lists and so on — and can be used to describe, to some degree, the appearance and semantics of a document. HTML's grammar structure is the HTML DTD that was created using SGML syntax.

Originally defined by Tim Berners-Lee and further developed by the IETF, HTML is now an international standard (ISO/IEC 15445:2000). Later HTML specifications are maintained by the World Wide Web Consortium (W3C).

Early versions of HTML were defined with looser syntactic rules which helped its adoption by those unfamiliar with web publishing. Web browsers commonly made assumptions about intent and proceeded with rendering of the page. Over time, the trend in the official standards has been to create an increasingly strict language syntax; however, browsers still continue to render pages that are far from valid HTML.

XHTML, which applies the stricter rules of XML to HTML to make it easier to process and maintain, is the W3C's successor to HTML. As such, many consider XHTML to be the "current version" of HTML, but it is a separate, parallel standard; the W3C continues to recommend the use of either XHTML 1.1, XHTML 1.0, or HTML 4.01 for web publishing.

There is no official standard HTML 1.0 specification because there were multiple informal HTML standards at the time. However, some people consider the initial edition provided by Tim Berners-Lee to be the definitive HTML 1.0. That version did not include an IMG element type. Work on a successor for HTML, then called "HTML+", began in late 1993, designed originally to be "A superset of HTML…which will allow a gradual rollover from the previous format of HTML". The first formal specification was therefore given the version number 2.0 in order to distinguish it from these unofficial "standards". Work on HTML+ continued, but it never became a standard.

The HTML 3.0 standard was proposed by the newly formed W3C in March 1995, and provided many new capabilities such as support for tables, text flow around figures, and the display of complex math elements. Even though it was designed to be compatible with HTML 2.0, it was too complex at the time to be implemented, and when the draft expired in September 1995 work in this direction was discontinued due to lack of browser support. HTML 3.1 was never officially proposed, and the next standard proposal was HTML 3.2 (code-named "Wilbur"), which dropped the majority of the new features in HTML 3.0 and instead adopted many browser-specific element types and attributes which had been created for the Netscape and Mosaic web browsers. Math support as proposed by HTML 3.0 finally came about years later with a different standard, MathML.

HTML 4.0 likewise adopted many browser-specific element types and attributes, but at the same time began to try to "clean up" the standard by marking some of them as deprecated, and suggesting they not be used.

Minor editorial revisions to the HTML 4.0 specification were published as HTML 4.01.

The most common extension for files containing HTML is .html, however, older operating systems, such as DOS, limit file extensions to three letters, so a .htm extension is also used. Although perhaps less common now, the shorter form is still widely supported by current software.

Markup element types
Below are the kinds of markup element types in HTML.

Structural markup. Describes the purpose of text. For example,
Golf
directs the browser to render "Golf" as a second-level heading, similar to "Markup element types" at the start of this section. Structural markup does not denote any specific rendering, but most web browsers have standardised on how elements should be formatted. For example, by default, headings like these will appear in large, bold text. Further styling should be done with Cascading Style Sheets (CSS).
Presentational markup. Describes the appearance of the text, regardless of its function. For example,
boldface
will render "boldface" in bold text. In the majority of cases, using presentational markup is inappropriate, and presentation should be controlled by using CSS. In the case of both bold and italic there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely strong emphasis and emphasis respectively. It is easier to see how an aural user agent should interpret the latter two elements. Note that most presentational markup elements have become deprecated under the HTML 4.0 specification, in favour of CSS based style design.
Hypertext markup. Links parts of the document to other documents. For example,
javvin
will render the word javvin as a hyperlink URL.