This section is the chapter's lookup shelf. You will come back to it when a validator complains about an unescaped ampersand, when a server answers 415, or when an interviewer asks why PUT is idempotent and POST is not.
The first five subsections cover markup; the rest cover URL encoding, MIME types, media queries in attributes and the HTTP methods, headers and status codes your pages travel with. Tables list the entries you meet in practice.