There is significant tearing in the graphics when watching videos in the HTML5 client on Firefox. Tested with ThinLinc 4.7.0 and Firefox 50 on Fedora 25.
This is a result of how we draw things in the HTML5 client. This is fixed in upstream noVNC with the implementation of double buffering.
The double buffering has gotten rid of almost all tearing. It's not completely gone though, but it's good enough for now.
There is a big improvement using double buffer mainly seen when using firefox, but also noticeable using chrome.
Works as expected.