My HTML does not work: the 8 most common mistakes

1. Unclosed tag: <p> opened with no </p> — the rest of the page breaks.
2. Wrong nesting: write <b><i>…</i></b>, not <b><i>…</b></i>.
3. File name: a hidden .txt, like index.html.txt (show file extensions in Windows).
4. Image path and case: Cat.jpg ≠ cat.jpg.
5. Missing quote: href="a.html (no closing quote).
6. Encoding: no <meta charset="utf-8"> — letters like ə, ö, ş break.
7. The browser shows an old copy: hard-refresh with Ctrl+F5.
8. You did not save the file (Ctrl+S).

Press F12 in the browser and open Elements: it shows how the browser understood your HTML. For an official check use validator.w3.org.