Nice UI; I really like the timeline view!

I would highly recommend adding .webm export (since Browser-based .webm encoding strongly correlates with the ScreenCapture API). If you'd like an example, you can reference my take on this: https://gifcorder.com/

There's also an Emscripten implementation of Gifsicle which you can use to reduce the size of the .gif (at the cost of color depth and encoding time).