I haven't upgraded to El Capitan, and may not for a while, but the latest version of Safari is 9.0, and is available for OS X Mavericks, aka 10.9. I upgraded to stay current with the security fixes, and have been pleasantly surprised to see a much improved Web Inspector.
Chrome and Firefox are probably the gold standard for friendly web developer tools, but I still use Safari all the time. It's the best option for debugging sites on iOS using the remote debugging features. You open your site in Safari running in the iOS simulator, and in the Develop menu, like this:
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