Close HTML tags as soon as one opens them

I'd like the corresponding, closing HTML tag to be automatically inserted whenever I open one.

So if I type

<div>

I should get

<div></div>

Without having to call to sgml-close-tag myself.

How to achieve this?

Solution

Rather than calling a hook function after every single key-stroke, it makes sense to only call it after a > was typed. This can be achieved by rebinding the > character in the keymap that sgml-mode uses.

In addition, sgml-close-tag shouldn't get called if the tag is already closed. Therefore, the following code adds a simple regexp check for that:

(defun my-sgml-insert-gt ()
  "Inserts a `>' character and calls 
`my-sgml-close-tag-if-necessary', leaving point where it is."
  (interactive)
  (insert ">")
  (save-excursion (my-sgml-close-tag-if-necessary)))

(defun my-sgml-close-tag-if-necessary ()
  "Calls sgml-close-tag if the tag immediately before point is
an opening tag that is not followed by a matching closing tag."
  (when (looking-back "<\\s-*\\([^</> \t\r\n]+\\)[^</>]*>")
    (let ((tag (match-string 1)))
      (unless (and (not (sgml-unclosed-tag-p tag))
           (looking-at (concat "\\s-*<\\s-*/\\s-*" tag "\\s-*>")))
    (sgml-close-tag)))))

(eval-after-load "sgml-mode"
  '(define-key sgml-mode-map ">" 'my-sgml-insert-gt))