The rightmost button on the tool palette is the Preview button, which previews the current document using your favorite WWW browser.
Clicking on the Preview button saves the current HTML document as “HTML.edit.temp.html”, then opens your WWW browser as specified in the Editor Preferences.
Note that some browsers also allow you to set the application used for viewing the HTML source of the currently viewed document. Set this preference to HTML.edit to use it to view and edit documents found online.