This very much depends on the bugs. In the past I've found many bugs in text rendering (especially related to non-English text[1][2]), layering (inconstent opacity), audio and video playback, etc. which were dependent on the combination of browser, operating system, and, in some cases, hardware. The general trend has been for browsers like Chrome and Firefox to take over more responsibility but that's not what Apple has done in Safari so it looks like a lot more work to say they should duplicate OS functionality to make it easier for Windows-only web developers to test with Safari.

1. https://chris.improbable.org/experiments/browser/combining-h...

2. https://chris.improbable.org/experiments/browser/javanese-te...