Progressive enhancement starts with clean, semantic HTML that works and remains understandable even without CSS or JavaScript, then adds visual styling and interactive behavior on top as enhancements rather than requirements. If a script fails to load or a user’s assistive technology cannot process some advanced feature, the core content and functionality still work.
This approach tends to produce naturally more accessible, more resilient sites than the opposite strategy of building a JavaScript-dependent interface first and trying to patch in accessibility and fallback support afterward, which is considerably harder to retrofit successfully.