On a fresh installation of Windows 10, I found that Youtube was working just fine, but other embedded videos on sites like Twitch and Reddit were not. I thought it had something to do with HTML5 or flash, but as it turns out, it was because I had chosen the wrong version of Windows during the installation process. Choosing Windows Professional N or, basically any N version, results in video issues primarily due to the lack of appropriate codec and DRM support. Even Netflix will very likely not play. This also affects all browsers. While I tried installing a bunch of codecs to try to fix this, once I learned that I would also encounter DRM issues (i.e., Netflix etc. won't work), I decided to pack it in and just reinstall the correct version of Windows.
Other possible fixes that the Google Gods spat out at me were to do with GPU issues. Turning off hardware acceleration in your browser, enabling software acceleration in Internet options were a couple of the suggested fixes. But they did FA for me.
Hope this helps someone out there.