What is the !important CSS rule?

CSS tries to create a balance of power between author and user style sheets. By default, rules in an author's style sheet override those in a user's style sheet.

If you, in your style sheet want to mark something as important, use the following code:

P { font-size: 18pt ! important }

You are telling your user, that it is important, that you use 18pt, as your fontsize.