See https://github.com/w3c/csswg-drafts/issues/2366

TL;DR - the spec doesn't currently say what context to use for parsing the CSS values in the "procedure to process a keyframes argument". Currently we use target->GetDocument() if it is available, or if there is a null target we use *ToDocument(ExecutionContext::From(script_state)).

If and when the spec decides what context is correct, we should make sure our code matches. And add a test!