"create navigation params by fetching" blocks about:blank and data: iframes under COEP #13086

What is the issue with the HTML Standard?

Found this by implementing to spec and hitting test failures in:

• https://wpt.fyi/results/html/cross-origin-embedder-policy/non-initial-about-blank.https.html
• https://wpt.fyi/results/html/cross-origin-embedder-policy/data.https.html

Which all engines pass.

Issue appears to be 21.13 of https://html.spec.whatwg.org/multipage/browsing-the-web.html#create-navigation-params-by-fetching where if the parent document has COEP enabled, this check blocks about:blank and data: responses. They have no CORP header, so the check treats them as same-origin, and their opaque origin never matches. As a result, the parent can’t navigate an iframe to data:, or to about:blank after the initial one.