Hi,
This tutorial is about having chromium working with 720p videos on websites like youtube.
Step 1 : Activate opengl driver
Follow my tutorial here : [Tutorial] - Activate OpenGL driver for ubuntu mate 16.04
Step 2 : Install Chromium
Chromium package from repository don't work at this day (June 2016), you must follow this tutorial to install it : [Tutorial] - Install working chromium 50
Step 3 : Configure Chromium
Edit /etc/chromium-browser/default and remove "disable-gpu" from CHROMIUM_FLAGS if there is one.
For editing, you must be root :
sudo pluma /etc/chromium-browser/default
Launch chromium and type in url :
chrome://flags
Search option "Media Source API" (#disable-media-source) and disabled it.
Close Chromium and restart it !
Go to youtube : https://www.youtube.com/watch?v=wyx6JDQCslE
There is only two resolutions 360p or 720p, it is because of disabling media source api.
If media source api is enabled video stops a lot of time (every few seconds).
720p videos works really great on my rpi 3 on windowed or full screen !
Rendering is software only but works well !
If you have audio delay or not sync with video ask me, I have maybe a solution 
Enjoy,
Aworan