doctype

This option enables you to specify the doctype that is used while editing content within TinyMCE.

The default value is set to:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

This option does not affect the format of tags: "<br>" versus "<br />". See element_format option instead.

Note however that there is no need to change the doctype to XHTML because TinyMCE will produce XHTML in any case. Read the comments here [1].

On the other hand, if you set it to XHTML your IE7 users will have problem selecting the edit area.

If unsure, don't specify the doctype.

Example of usage of the doctype option:

tinyMCE.init({
	...
	doctype : '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">'
});