User blog comment:Wildoneshelper/15 Things You Don't Know About Editing/@comment-4189499-20140813231605

The reason for number 2 is that it's safer for open-editing websites to not be able to use all tags to prevent viruses or other nasties being added. That's why MediaWiki software prevents the use of certain tags such as a, button, and marquee. However, all tags can be used in the MediaWiki namespace, which only admins can edit, but once something is placed on a page in the MediaWiki namespace it can be transcluded to other pages using tags, therefore meaning that all tags can technically be used on Wikia, but it requires some work-arounds.