Wanna-be to sgml-mode as psgml-edit used to be to psgml, back in the day.
For now, there's just that one function sgml-split-element. If you want to
bind it to C-c RET, add this to your .emacs:
(eval-after-load "sgml-mode"
'(progn
(require 'sgml-edit)
(define-key html-mode-map "\C-c\r" 'sgml-split-element)))
That is all.