This error fundamentally cannot block your app’s CSS when your app is actually loaded (to be shown on screen).
What’s happening here is that we use a sandboxed iframe to preload your app before loading it for real in a different iframe. The preloading sandbox iframe intentionally prevents JS from loading (which results in those “errors” in your console), but that frame is never shown on screen. It is removed the moment your app begins loading for real.
Your app is loaded in a totally separate iframe that does not have sandboxing enabled, and there has been no change to that configuration. The change here is that your app’s subresources (JS, CSS, fonts) are now usually loaded from disk cache, which is populated by that sandboxed preload frame.
We’re exploring less “loud” solutions to this, but the reality is those browser “errors” are actually warnings. If we could silence them we would, but there’s no way to do so.