Small emacs lisp snippet to open email as HTML in default browser. Found it somewhere over the net, but it is so awesome I need to save it for posterity. In a summary buffer use v h (think of it as "view html") to switch to HTML view. It works by generating a temporary file and stripping the html part of the current message into it.

view_html.el

(defun my-gnus-summary-view-html-alternative ()
      "Display the HTML part of the current multipart/alternative MIME message
    in current default browser."
      (interactive)
      (save-current-buffer
        (gnus-summary-show-article)
        (set-buffer gnus-article-buffer)
        (let ((file (make-temp-file "html-message-" nil ".html"))
              (handle (nth 3 (assq 1 gnus-article-mime-handle-alist))))
          (mm-save-part-to-file handle file)
          (browse-url (concat "file://" file)))))

 (define-key gnus-summary-mode-map "vh"
      'my-gnus-summary-view-html-alternative)