As per https://github.com/whatwg/dom/pull/589 give Range and StaticRange a common superclass. web-platform-tests changes: https://github.com/w3c/web-platform-tests/pull/9967.
FYI and FWIW, I think this is the single only feature in the DOM spec that currently isn’t yet implemented in at least two engines.
<rdar://problem/64231680>
*** This bug has been marked as a duplicate of bug 217846 ***