Thread:Project:Support desk/Extention: to take chunks of text and add html tags

What i want to do, is to take text, and add tags around it. Like i take a word "red" and color it red, without touching the wiki source that the user inputs. Without bypassing the caching schemes of mediawiki. I was thinking to take the final raw html, and work from there. How do i do that? From an extension of course.