text/html

The one type browsers are really built around, registered and maintained by the HTML standard itself. The standard requires authors to use UTF-8, but there is no UTF-8 default on the wire — leave the charset undeclared and browsers fall back to legacy guessing, usually windows-1252. Declare it in the Content-Type header or in a meta tag inside the first 1024 bytes. Serving XHTML as text/html is fine and universal; the document simply gets the forgiving HTML parser instead of the strict XML one.

.htm is the DOS-era 8.3 spelling; the two are interchangeable.

Defined by HTML Standard — IANA considerations · IANA registration. Registry facts from the IANA media-types registry via mime-db.