https://html.spec.whatwg.org/multipage/webappapis.html#processing-model-9 says:
1. Let now be the value that would be returned by the Performance object's now() method 2. Let docs be the list of Document objects associated with the event loop in question… ... 4. For each fully active Document in docs, run the resize steps for that Document, passing in now as the timestamp ... This makes no sense, as performance.now() is per-document (it’s relative to the document start time), so passing the same value to all documents in the browsing context is bogus. What may be intended is to “freeze” the performance.now() time in all documents before processing those documents, but give each document its own performance.now() time. Simon
