I second Limbo here, why do you want to do this? You really need to examine why you think this is required before you decide to do it.
Perhaps using existing tags with Mircoformats would be better?
Most browsers will render any tag you want. Except older versions of IE of course. For them you'll need to do something similar to what the HTML5 shiv does for HTML5 tags which creates tags that the brower doesn't know about via JavaScript.