Safari 10.0.3
Apple has released Safari 10.0.3 for OS X 10.10.5 Yosemite and 10.11.6 El Capitan (macOS 10.12.3 Sierra also includes it; see “Apple Releases macOS Sierra 10.12.3, iOS 10.2.1, tvOS 10.1.1, and watchOS 3.1.1,” 23 January 2017) with patches for WebKit-related security vulnerabilities. The update deals with multiple memory corruption issues that could allow arbitrary code execution after processing maliciously crafted Web content, plus a state management issue that could lead to address bar spoofing. Safari 10.0.3 is available only via Software Update. (Free, release notes, 10.10+)
I keep a copy of Opera on my macbook Pro for when (frequently) Safari goes "spinning beachball"
I click on update and the when computer tries to restart I get a box that says 'update details have changed' when I click to see, I get back to the original update screen in the App Store. This I can repeat, going round the circle, for ever it seems.