No one said that you can not use both. What is not recommended is using both styles in one document. Purely to stop confusion.

You can understand why they are insitsting or pushing it though. They want people and browsers to adopt a new CSS standard. How are they going to manage that if people are still stuck with HTML 3.2? By forcing people to use a new standard it helps to promote that standard and hopefuly get it recognised by future web browsers.

- wil