I'm not sure they can contribute their improvements terribly easily. Apple changed their development policies recently to be fairly hostile to non-Apple users of Webkit; basically, developers are allowed to check in changes that break the build on non-Apple platforms (which is enough to make development elsewhere a pain on its own because it breaks git bisect), and commits - including ones to non-Apple platform code, even ones that fix the build - now require the approval of an Apple employee with no knowlege of other platform and no incentive to approve changes important to them promptly.
Relax, I'm sure someone will come along and explain that it's not anti-competitive and Apple have everyone's best interests in mind.
It may be a balm to your soul to pretend that Apple is being especially evil here but they aren't the only ones doing this. I was recently given the task of compiling MySQL on AIX 7. It's doable but the code is riddled with stuff committed by people who don't care what else they are breaking as long as it works on Linux. Not that I'm complaining, AIX versions newer than 5.3 are not supported by the MySQL team due to "low demand" and I can easily sympathise with the MySQL team's reluctance to support it. It's impossible to maintain a complex software project and support Windows, the Unixes, a vast collection of fragmenting Linux distributions, Android, iOS, OS X.... (the list goes on). Sooner or later the communities surrounding these OS'es are going to have to step up and fix the broken code if they want the software to be supported on their OS. You can't expect Apple, Oracle, Google, et al... or non profit FOSS efforts for that matter, to support every single OS out there and all of their variants.