hide-all-tags.el

(defun toggle-tags-invisible ()
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (when (not (remove-text-properties (point-min) (point-max) '(invisible t)))
      (let ((spew (make-progress-reporter "Hiding all tags..." (point-min) (point-max))))
        (while (re-search-forward "</.*?>" nil t)
          (add-text-properties (match-beginning 0) (- (match-end 0) 1) '(invisible t) nil)
          (progress-reporter-update spew (point)))
        (goto-char (point-min))
        (while (re-search-forward "<[a-z].*?>" nil t)
          (add-text-properties (+ 1 (match-beginning 0)) (match-end 0) '(invisible t) nil)
          (progress-reporter-update spew (point)))
        (progress-reporter-done spew)))))

(add-hook 'nxml-mode-hook
          (lambda ()
            (visual-line-mode)))

(font-lock-add-keywords
 'nxml-mode `(("<[a-z].*?>"
               (0 (progn (compose-region (match-beginning 0)
                                         (match-end 0) "\u27e6")
                         nil)))
              ("</.*?>"
               (0 (progn (compose-region (match-beginning 0)
                                         (match-end 0) "\u27e7")
                         nil)))))