HTML5 (Hypertext Markup Language 5) is a markup language used for structuring and presenting hypertext documents on the World Wide Web. It was the fifth and final major HTML version that is now a retired World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML Living Standard. It is maintained by the Web Hypertext Application Technology Working Group (WHATWG), a consortium of the major browser vendors (Apple, Google, Mozilla, and Microsoft).

HTML5 was first released in a public-facing form on 22 January 2008, with a major update and "W3C Recommendation" status in October 2014. Its goals were to improve the language with support for the latest multimedia and other new features; to keep the language both easily readable by humans and consistently understood by computers and devices such as web browsers, parsers, etc., without XHTML's rigidity; and to remain backward-compatible with older software. HTML5 is intended to subsume not only HTML 4 but also XHTML1 and even the DOM Level 2 HTML itself.

HTML5 includes detailed processing models to encourage more interoperable implementations; it extends, improves, and rationalizes the markup available for documents and introduces markup and application programming interfaces (APIs) for complex web applications. For the same reasons, HTML5 is also a candidate for cross-platform mobile applications because it includes features designed with low-powered devices in mind.

Many new syntactic features are included. To natively include and handle multimedia and graphical content, the new <video>, <audio> and <canvas> elements were added; expandable sections are natively implemented through <summary>...</summary> and <details>...</details> rather than depending on CSS or JavaScript; and support for scalable vector graphics (SVG) content and MathML for mathematical formulas was also added. To enrich the semantic content of documents, new page structure elements such as <main>, <section>, <article>, <header>, <footer>, <aside>, <nav>, and <figure> are added. New attributes were introduced, some elements and attributes were removed, and others such as <a>, <cite>, and <menu> were changed, redefined, or standardized. The APIs and Document Object Model (DOM) are now fundamental parts of the HTML5 specification, and HTML5 also better defines the processing for any invalid documents.

The Web Hypertext Application Technology Working Group (WHATWG) began work on the new standard in 2004. At that time, HTML 4.01 had not been updated since 2000, and the World Wide Web Consortium (W3C) was focusing future developments on XHTML 2.0. In 2009, the W3C allowed the XHTML 2.0 Working Group's charter to expire and decided not to renew it.

The Mozilla Foundation and Opera Software presented a position paper at a World Wide Web Consortium workshop in June 2004, focusing on developing technologies that are backward-compatible with existing browsers, including an initial draft specification of Web Forms 2.0. The workshop concluded with a vote—8 for, 14 against—for continuing work on HTML. Immediately after the workshop, WHATWG was formed to start work based upon that position paper, and a second draft, Web Applications 1.0, was also announced. The two specifications were later merged to form HTML5. The HTML5 specification was adopted as the starting point of the work of the new HTML working group of the W3C in 2007.

WHATWG's Ian Hickson (Google) and David Hyatt (Apple) produced W3C's first public working draft of the specification on 22 January 2008.

Many web browsers released after 2009 support HTML5, including Google Chrome 3.0, Safari 3.1, Firefox 3.5, Opera 10.5, Internet Explorer 9 and later.