Both nxml-mode and html-mode obey comment-padding, though slightly differently. nXML defaults to no spacing (like nXhtml); nXHTML seems to replace the first character of comment-padding with a space. So, for example:
(setq comment-padding "xy")
<!-- yy --> - html-mode
<!--xyxy--> - nxml-mode
It seems to be only nXhtml-mode that inserts <!----> regardless.
Both nxml-mode and html-mode obey comment-padding, though slightly differently. nXML defaults to no spacing (like nXhtml); nXHTML seems to replace the first character of comment-padding with a space. So, for example:
(setq comment-padding "xy")
<!-- yy --> - html-mode
<!--xyxy--> - nxml-mode
It seems to be only nXhtml-mode that inserts <!----> regardless.