Opera Mini has been updated with the ability to download videos for offline viewing. Unfortunately, YouTube videos are not supported, probably due to restrictions imposed by Google. Microsoft made a YouTube client of its own for Windows Phone, which Google called for its removal from the Windows Store because it included a download tool. I assume Opera stopped short of adding this feature to avoid Google's wrath.
Uploaded videos can be downloaded from social media or other video platforms and saved to the device internal storage and SD card. Check out the new feature in Opera Mini latest update in Google Play store. Watch the video below to see how it all works: