(X)HTML-mode

Normally Emacs enters html-mode when it opens a file whose name ends in ".html" or ".htm". You can also switch to this mode by invoking the command html-mode.
The file is opened in xhtml-mode if it contains an XML declaration, usually the first line:

<?xml version="1.0" encoding="iso-8859-1"?>

See http://en.wikipedia.org/wiki/XML for XML (Extensible Markup Language).

In this mode there are two new menus, HTML and SGML.

To see the description of the mode, do C-h m.

Here are a few more often used commands. For most of them the completion mechanism works.

See this link at https://www.gnu.org/software/emacs/manual/ for more [as of 2014.10].

C-c C-t

purpose: insert a tag (you can choose from the tags offered or type a new one); for some tags it will also help you add attributes.
remarks: if there is a highlighted region, the tag/end-tag is wrapped around the region.

C-c /

purpose: close the currently open tag

C-c C-d

purpose: remove a tag, including the attributes and the end-tag.
remarks: the point should be inside the < and > defining the tag or the end-tag.

C-c C-a

purpose: add attributes; you can choose from those offered, or type a new one (for some attributes you are able to input the value too).
remarks: the point should be inside the < and > defining the tag; the attributes are inserted where the point is.

C-c C-v

purpose: open the current buffer in a browser (without worrying about the read-permission).
remarks: the next time this command is invoked, the buffer is reloaded in the same browser window.

comment-region

purpose: comment and uncomment the region
remarks: to uncomment the region use uncomment-region (or, precede the command with C-u, i.e. type C-u M-x comment-region).

C-c C-i, same as C-c TAB

purpose: Toggle visibility of existing tags