XHTML without submitting

I am using the editor without submitting the data by post, but rather by using javascript. For instance, the editor is a popup that a user can use if they do not want to use a standard textarea, the popup then will be used to format the text, and then a "make changes" button will send the formatted text to the opener window.

This is working fine, but I want the formatted text to be XHTML compatible, and im not sure what js function to call to do that, any ideas?

Many thanks

John