Progressive enhancement is a strategy in web design that puts emphasis on web content first, allowing everyone to access the basic content and functionality of a web page, whilst users with additional browser features or faster Internet access receive the enhanced version instead. Additionally, it speeds up loading and facilitates crawling by web search engines, as pages' text is loaded immediately through the HTML source code rather than having to wait for JavaScript to initiate and load the content subsequently, meaning content ready for consumption "out of the box" is served imminently, not behind additional layers.

This strategy involves separating the presentation semantics from the content, with presentation being implemented in one or more optional layers, activated based on aspects of the browser or Internet connection of the client. In practice, this means serving content through HTML, the "lowest common denominator" of web standards, and applying styling and animation through CSS to the technically possible extent, then applying further enhancements through JavaScript. Deprecated Adobe Flash could be thought of as having shared the final spot with JavaScript while it was widely in use. Since web browsers can load site features to the extent supported rather than failing to load the entire site due to one error or missing feature in JavaScript, a progressively enhancing site is more stable and backwards compatible.