Headings are vitally important when assisting in scanning and navigating your web page (see LevelAccess's simulation of Web Headings for Screen Readers). They also help with search engine optimization (SEO), letting search engines know what topics your web page contains.

Headings do not stand alone; they introduce subject matter. Headings should not be used to make paragraphs stand out, as that makes the content unscannable by your constituents. They should be no longer than 140 characters.