See https://github.com/whatwg/html/pull/6567 and the tests at https://github.com/web-platform-tests/wpt/pull/28403 , which Firefox and Chrome already pass.
<rdar://problem/76905573>
Safari is only one failing this test case: https://wpt.fyi/results/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/remove-initial-about-blankness.window.html?label=experimental&label=master&aligned Just wanted to update status. Thanks!
We're passing this test case now.