This is just a word of warning to any people wanting to update to Stretch any time soon:
It seems that the newest version of Chromium that Stretch uses has some issues when using the H.264ify extension, i.e: it crashes any time it tries to load a video. I spent ages trying to figure this out, since even disabling the extensions didn’t help. I had to completely remove the extension to get Chromium working properly again.
Even without the extension, the newest Chromium version plays videos perfectly fine, so if you come across any inexplicable “Oh snap” warnings, you’ll need to remove the extension.
Hope this is useful! :D