If using Chrome:
1. Open a new tab and enter “chrome://flags/”.
2. Press ctrl F to search for “disable hardware-accelerated video decode”.
3. If it is reading as “disabled”, click the “enabled” button.
4. Scroll to the bottom and click on “relaunch now”.
There you go! Pass this on in case your Chrome browser ever resets itself to disable videos on Tumblr, or keep this in your drafts for future reference.