You can view the HTML source for a page, which is useful if you're familiar with HTML. However, mistakes can cause your browser to crash or make your page unable to be displayed.
As a precaution, the wiki allows specific HTML tags and attributes. This helps prevent people from adding malicious HTML source. If you add HTML source and notice it removed or reformatted, it might not be allowed.
To edit HTML:
While editing a page, click the Edit HTML (<>) button.
Edit the HTML source and then click the Edit HTML button again to verify your changes.
Your changes are verified for validity and disallowed HTML is removed. If the page doesn't appear, the edited HTML isn't allowed.