Hi! This is great, we have waited this feature for many years! 
I tested the latest commit from this branch: manager-msaa (https://github.com/CesiumGS/cesium/tree/manager-msaa)
I experience a bad performance bug still: on a machine with Nvidia RTX 3090 GPU even the empty sandcastle fullscreen runs about 25 fps. With scene.msaaSamples=1 fps is way above 60fps all the time. From my understanding this cannot be just normal performance penalty of MSAA, it has to be some error somewhere…