Spec change: https://github.com/whatwg/html/pull/3472
Tests: http://w3c-test.org/html/semantics/forms/textfieldselection/selection-start-end-extra.html

Note that these tests do not include many ways to change the value that are only possible manually, e.g. copy and paste or user input. See https://github.com/whatwg/html/issues/2424#issuecomment-286580627 and https://github.com/whatwg/html/issues/2424#issuecomment-286605947 for a more exhaustive list.

We get most of the tests linked above correct, but might fail other cases.