I don't know how much pull Safari has. Seems like a lot of web devs don't test on Safari.
I know this isn't damning evidence or anything, but I noticed recently that the header image on front page of Thunderbird's homepage[1] has an image of the app (in a Mac window, funnily enough) that doesn't size properly on Safari[2]. It squishes horizontally, I assume due to some weird CSS implementation difference.