The security issue really should be front and center in a project like this. It's kind of disappointing to see it left as an exercise for the embedder.

In the general case with untrusted 3rd party code the only really safe solution is to embed the blocks in cross-origin iframes. In the face of Spectre and Meltdown you really want those to be out-of-process iframes too, but that's up to the browser.

iframes are cumbersome of course, so it would have been interesting to see the protocol focus on making them more seamlessly integrate with the embedding context with a standard postMessage protocol for things like securely forwarding content sizes (for auto-resizing the frame), events, and styling information across the frame boundary.