Emacs as a web browser's external editor
Since text editing in Emacs is better, extensions are developed for all major web browsers to use Emacs as their external editor. Mostly these extensions make use of Emacs daemon.
Iceweasel, Conkeror etc.
Conkeror comes with a small C program that enables external editing of form fields and page source in Emacs. Thus, no third-party extension/plugin is required.
Safari, Chrome etc.
Older versions of Opera based on Presto had a version of the popular Firefox It's All Text extension. Newer versions of Opera based on Chromium do not have a native extension (yet). However, Opera does support Chrome/Chromium extensions to some extent via this extension. You can then install the "Edit with Emacs" extension from the Chrome Web Store.