We're planning to ship full masking support (+ animations, naturally) within the coming quarter.
In the meantime, you can simulate a mask by creating a full-stage layer that's the same color as your expected (host codebase, embedding) background, but with a hole punched out. You can then animate elements underneath, executing a mask.
This doesn't work for every scenario, but sometimes it's good enough.
Here are an example you can fork and play with!