From the commentary:

Provides a dummy major mode called html-chooser-mode that looks at a file, and if it seems to be XHTML, switches to nxml-mode, and if it seems to be regular HTML, switches to html-mode (or html-helper-mode, or whatever). If the file is empty or otherwise cannot be understood, the user is prompted to choose which mode, and is offered the possibility of inserting several skeletons or templates appropriate to that mode.