Styling Open-ended Text Inputs

What's the best / recommended practice to treat a text input that contains HTML tags? The idea is to properly display the input e.g. on a different page. As expected, people can be

Solution 1:

As rlb.usa noted in the comments, most people rely on a mature, well-tested HTML renderer to display HTML. WebKit, Qt, and Microsoft's Trident control are popular choices depending on the platform.

If you've got a renderer that has problems with nonstandard code, you could run it through something like tidy first.