When you code something, choose the most strict browser/compiler you can. Then you get it right the first time and don't have to see it break so much later when tested elsewhere.
Which do you use? I've been bitten numerous times by a seemingly normal object or array method being Mozilla only or not yet standardised...
Funny thing: mobile Safari is a deeply frustrating browser to work with simply because it lacks proper full screen support and is very twitchy about showing/hiding its chrome (problematic for my use case). However, desktop Safari does often seem to nail the standard, at least for the APIs I use most.