Hey, thanks for trying Jetpack Boost out!
Jetpack Boost uses an invisible iframe to generate Critical CSS for your site. That iframe is configured with strict sandboxing rules for a few reasons; to help protect your admin dashboard from unexpected script execution, to ensure that scripts for tracking page views aren’t triggered (so we don’t mess up your stats), and to ensure that the Critical CSS we generate reflects the initial page-load content without external scripts affecting the layout.
Unfortunately, browsers tend to log those messages to the console to let developers know that resources are being blocked by the iframe’s sandboxing rules.
They aren’t a signal that anything has gone wrong; they just indicate that your browser is correctly enforcing the iframe sandboxing rules.
