I was having problems with various sites using HTML5’s video features (webm! mp4!) simply not working on Android’s Chrome browser. Most noticeably Gfycat and Streamable, both heavily used in sport related subreddits. Trying to start a video of either site would invariably result in a black screen, maybe the video loading in the background, but never actually playing. A “solution” was to then try and download the actual video so I could play it via VLC. Rather annoying if you just want to quickly view some amazing play and swiftly leading to simply not bothering with the broken links at all.
After some looking around and playing with settings, I found something that did the trick and fixed everything for me. Go into your settings, the developer options category. If you do not have this category, check out something like this page to enable it as it is hidden by default. Once there, locate the setting regarding NuPlayer or AwesomePlayer. In my case the setting was “Use AwesomePlayer (deprecated)”, which I turned to off. Apparently it can also be called “Use NuPlayer (experimental)” in which case you want it turned on. Think of it as not wanting AwesomePlayer.
With that done, videos should be playing correctly! Note that you may need to restart Chrome for it to take effect.