>>87
I don't want to be the guy, but you shouldn't be troubleshooting by committee when you have custom CSS in play. Every modern browser runs a video an internal video handler and things like controls and scrubbing bars can be (and are) overridden and often hidden and replaced with a wholly HTML+CSS overlay. Youtube does this. You can, if you want, hide their custom control overlay and unhide the default browser controls.
This is in exception to things like flash and silverlight players, but the present HTML5 video design is an intent to obsolete plugin-based video playback.
Either way, if you are messing with CSS, you are messing not only with styling but very often the underlying layout structure of the page. And that can mean the video player.
Disable your tampermonkey script -- probably in a separate tab -- and see if the issue persists. If it does, then that is right wonky shit right there. If not, then you need to be looking at whether or not your CSS overrides are targeting more than the elements you think they are. Again, and this bears repeating, player controls are no different than the markup in the rest of the page. If you're watching through Cytube/8chan's embedded player (as opposed to capturing the stream or something), then you are definitely looking at a HTML5 <video> tag with custom overlays.