Bug 66564 is about what WebKit's behavior should be wrt. the device scale factor when the WebView/WKView isn't in a window. One special case of this is when the view isn't in a window but it does have a host window. In that case, we could choose to use the backing scale factor of the host window. This would have the advantage of having window.devicePixelRatio return the same value for all tabs in a Safari window. (Background tabs have a host window but aren't in a window.) The only reason I can think of why we wouldn't want to do this is because it would mean we wouldn't match AppKit's idea of the view's scale factor, and it's possible that could cause problems. But maybe it won't!
Note that WKView has no host window concept. We'd have to add one if we wanted to do this.
<rdar://problem/9987507>
This was addressed in Safari, not WebKit.