I therefore wanted to have a look at the performance of the animations (I'm not talking about WebGL here), so here it is; the video shows three samples (thanks to the authors!) run on each of these alternatives: WebKit 1 on raster, WebKit 1 on OpenGL, WebKit 2 and then QtWebEngine.
As you can see WebKit 1 on raster is completely unusable and WebKit 1 on OpenGL is clearly better, but still not very good. Also you can see how the pointer moves: the main thread is too busy and user interaction is difficult. WebKit 2 and QtWebEngine solve this issue and improve performance.