Very likely the restriction was placed there for a reason.
This reason is that unrestricted html code can mess up the html page.
Worse still, it may include javascript and other clientside code, which could be a security problem.
To allow html and yet remain secure, one would have to restrict the tags allowed, usually by using some sort of regular expressions.
Unfortunately, if you do not really know PHP (or regular expressions, for that matter), then it's going to be difficult.