download Open Broadcaster Software

Under "Scene" Add a new one named anything

-- Then, to the right under Sources -- the easiest way to do things is to add a "Monitor Capture" and set yourself a hotkey in the Settings to start and stop recording. Then you just play the video in full screen, press record when you want, and stop when you want. The video will show up as a .flv in your Videos. Upload it to YouTube, Dailymotion, Vimeo, Vidme, etc and voila you have your video

The more advanced way to do it -- instead of "Monitor Capture" add a "Window Capture", select the browser window, at the part that says "Sub-region" click yes, hit select region, make the region go directly around the video in the tacs window, then click "Preview Stream" in OBS and right click on the video -- go to Positon/Size --> Fit to Screen. Now you can do anything you want on your computer but your stream will only output the video from the browser window. It will still record other sounds. You can just hit stop and start recording in OBS if you want, or still use your hotkeys at this point.

If you have an nVidia card you can already do this with shadowplay

Screencapture software..

You mean laughing at that liberal pussy Bill Nye the "science" guy getting his ass handed to him by Nick?!