Dear Shiro,
it might help if you specify where in ILIAS you are working (Learning module etc.). Also note that HTML contents are possibly going to be disabled in some ILIAS page editor contexts in future and you cannot really prevent someone from downloading a video, if they have full control over their client. I get sick if people are doing crappy things to disable options users with special needs might require, as users will install silly browser plug insin response that promise to be able to download videos.
Sorry for not answering the question; you might need a plugin for that to work. Writing an ILIAS plugin isn't that hard but you have to copy thousands of lines of boilerplate code.