I turned this into a greasemonkey script: https://greasyfork.org/en/scripts/531780-universal-video-fix...

That can be configured to run automatically on every site, and it uses the MutationObserver API to automatically fix any newly added/modified <video> elements on the page. Seems to work on Snapchat and Instagram so far, but breaks some functionality. Op's chrome extension, with bespoke support for Instagram, likely offers a much better experience. This is just more universal.