In the Labs Workbench UI, we occasionally (in ~3 locations) use the let keyword.

The purpose of let is to isolate the scope of variables, typically inside of loops.
This has historically always been problematic in JavaScript using the more common var keyword (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let)

More research is needed as to when Safari 10 will become the standard, as it is the first version supporting use of the "let" keyword.
Safari is not very common, but it seems that Safari 10 is already more prevalent than 9.1 (according to browser usage statistics).

This ticket is complete when we discuss how best to address lack of browser support.
My inclination is to say ignore this, since Safari is uncommon in browser usage statistics.