I /think/ doing this should be almost trivial. The current setup looks like <window> <iframe remote> ---[process boundary]--- <remote content> We should instead be able to just do <window> <iframe remote src="proxy.html"> ---[process boundary]--- <iframe remote> ---[process boundary]--- <remote content> The proxy page would have no background/margins/borders/style and just contain an unstyled <iframe remote>. We would then have to add a reftest-content-proxy.js testing script that bounced messages between reftest.js and reftest-content.js.