Block autoplaying animated loops and mouseover videos on Reddit…
Add to the ‘Filters’ list of the popular uBlock Origin browser add-on, and save.
There is the ability in Reddit’s main User Settings to stop auto-playing videos. But “…when in the viewport” is the kicker on that setting. The only other option in User Settings is to ‘Reduce Animations’, rather than actually eliminate them. The above, plus a GIF blocker, makes absolutely sure that nothing moves on Reddit.