WYSIWYG editor stripping off tags

I realise that the WYSIWYG editor strips off some tags, such as head and html.

I was wondering why it does this. Is there a situation in which this is desirable, or is it just that this is what the code for the WYSIWYG that you've chosen to use does this?

If it's the latter, is there a plan to stop the tags getting stripped by amending the WYSIWYG code?

Thanks.