If you’re using Silverstripe and you’re an experienced web developer you’ll notice, at some point, that there are some objects you can’t add into the HTML window of the editor. It’s not Silverstripe causing this, it’s TinyMCE. There are a list of snippets you are allowed to add and anything else will be removed. It’s easy to fix this, since I know you want to. I have discovered two methods. The first I found searching the web, must be for an earlier version of Silverstripe cause the file location and name didn’t match my install.