HTML5 is the successor of HTML and XHTML which has been incubated as a mixture of features introduced by various specifications, along with those introduced by software products such as web browsers, those established by common practice, and the many syntax errors in existing web documents. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers, etc.). HTML5 is intended to subsume not only HTML 4, but XHTML 1 and DOM2HTML (particularly JavaScript) as well. Many features of HTML5 have been built with the consideration of being able to run on low-powered devices such as smartphones and tablets.

In particular, HTML5 adds many new syntactical features. These include the <video>, <audio>, <header> and <canvas> elements, as well as the integration of SVG content that replaces the uses of generic <object> tags. These features are designed to make it easy to include and handle multimedia and graphical content on the web without having to resort to proprietary plugins and APIs. Other new elements, such as <section>, <article>, <header> and <nav>, are designed to enrich the semantic content of documents. New attributes have been introduced for the same purpose, while some elements and attributes have been removed. Some elements, such as <a>, <cite> and <menu> have been changed, redefined or standardized. The APIs and document object model (DOM) are no longer afterthoughts, but are fundamental parts of the HTML5 specification. HTML5 also defines in some detail the required processing for invalid documents so that syntax errors will be treated uniformly by all conforming browsers and other user agents.

In addition to specifying markup, HTML5 specifies scripting application programming interfaces (APIs). Existing document object model (DOM) interfaces are extended and de facto features documented. There are also new APIs, such as:

  • The canvas element for immediate mode 2D drawing.
  • Timed media playback
  • Offline Web Applications
  • Document editing
  • Drag-and-drop
  • Cross-document messaging
  • Browser history management
  • MIME type and protocol handler registration
  • Microdata

Not all of the above technologies are included in the W3C HTML5 specification, though they are in the WHATWG HTML specification. Some related technologies, which are not part of either the W3C HTML5 or the WHATWG HTML specification, are as follows. The W3C publishes specifications for these separately.

  • Geolocation
  • Web SQL Database, a local SQL Database (no longer maintained).
  • The Indexed Database API, an indexed hierarchical key-value store (formerly WebSimpleDB).
  • Web Storage, a key-value pair storage framework that provides enhanced behaviour similar to Cookies but with larger storage capacity and improved API.
  • File API, Handle file uploads and file manipulation.
  • Directories and System. This API is intended to satisfy client-side-storage use cases not well served by databases.
  • File Writer. An API for writing to files from web applications.

A common misconception is that HTML5 can provide animation within web pages, which is untrue. Either JavaScript or CSS3 is necessary for animating HTML elements. Animation is also possible using JavaScript and HTML 4, and within SVG elements through SMIL, although browser support of the latter remains uneven as of 2011.