I've tried through steemit.com and video works, indeed.

Tried again on https://www.steemstem.io/#!/@mobbs/slow-motion-phone-cameras-are-amazing using Chromium and not working there, and displaying error: ''An error occurred. Please try again later. (Playback ID: cXP7RuO8us7taug9)'' However, youtube video in one of the comments works. Same using Firefox.