Add HTML5 Placeholder to form inputs

Backdrop really ought to have, or be able to have, placeholders for admin form inputs. More and more designers hide form labels and rely on placeholders to provide information in a clean manner.

Perhaps the easiest place to visual this working is on the User registration/login and password forms. Here's an example:

Whilst it's feasible to add placeholders per input field and form through your theme template.php file, surely we should be taking full advantage of HTML5's features and have this in core for all admin forms?