I'm currently testing my website using FF's RDM, I have HTML 5 video with two sources, one Webm and one MP4 encoded according to the mozilla dev pages' advice on codecs. The video, when clicking play or the video itself, advances one frame and pauses again under all User Agents bundled into Responsive Design Mode. If I click quickly and repeatedly it will eventually play properly. The thumbnail renders properly and the progress bar indicates that the video has loaded properly. The video works as intended on Desktop (Windows & Linux), my Pixel 4a (FF & Chrome), my friend's S20FE (One of the UAs in RDM), and StackOverflow doesn't seem to have any input on my code, so the one last thing to check is with people who know more about Responsive Design Mode.