Progressive Enhancement

Building a page in layers — meaningful HTML first, then CSS, then JavaScript — so that each layer improves the experience for the browsers that can use it and none is required for the page to work

The content and the core interaction exist in the HTML; scripting adds convenience, not existence. On this site every calendar, chart and client-only component has a server-rendered fallback, and a document is readable with scripting off — that is the practice, not a slogan.