HTML5

HTML5 is the fifth major revision of the HTML standard, and the version that turned the browser from a document viewer into an application platform. Beyond new semantic markup, it introduced native audio and video, canvas graphics, and a family of browser APIs — Web Storage, WebSockets, geolocation, and Server-Sent Events among them — that let developers build rich, connected experiences without plug-ins.

• Semantic markup - Elements like <header>, <section>, <article>, and <figure> give documents clearer structure and accessibility.
• Native media - The <audio> and <video> elements replaced proprietary plug-ins for playing media in the browser.
• Browser APIs - Server-Sent Events, WebSockets, and Web Storage brought real-time and stateful capabilities directly into JavaScript.
• Living standard - The versioned “HTML5” milestone has since folded into the WHATWG HTML Living Standard.

For API work, HTML5’s browser APIs are where a lot of real-time consumption happens — Server-Sent Events, for instance, is an HTML5 mechanism many streaming APIs rely on to push updates over a plain HTTP connection. It is also the layer that developer portals, interactive documentation, and API consoles are built on, so the quality of an HTML5-based experience often shapes a developer’s first impression of an API long before they read a single line of the OpenAPI contract.