Apple has released Safari 10.1.1 for OS X 10.10.5 Yosemite and 10.11.6 El Capitan (macOS 10.12.5 Sierra also includes it; see “Apple Releases macOS 10.12.5, iOS 10.3.2, watchOS 3.2.2, and tvOS 10.2.1,” 15 May 2017). The update is focused solely on patching security vulnerabilities, addressing memory corruption issues in WebKit that could lead to arbitrary code execution, preventing address bar spoofing with improved state management, and resolving logic issues that existed in the handling of WebKit container nodes. Safari 10.1 is available only via Software Update. (Free, 10.10+)
I'm currently working with a customer with a broken webkit (mail doesn't work nor safari, nor app store.. ) Chrome works - but seems like there's no fix but a reload...Heading over there with 10.12.5 reinstall- this is now Standard Apple Operating behavior- they need to fix bugs and their Apps (i have two open app issues that are acknowledged but unfixed through (now) 4 security iterations).