See http://w3c-test.org/html/semantics/forms/textfieldselection/selection-start-end-extra.html and https://github.com/whatwg/html/pull/3472 for the HTML Standard changes.
<rdar://problem/38601821>
Safari Technology Preview 155 passes all tests from below: http://w3c-test.org/html/semantics/forms/textfieldselection/selection-start-end-extra.html I am marking this as "RESOLVED CONFIGURATION CHANGE". Please reopen, if this is about more than above testcase. Thanks!