[dom] getElementsByTagName needs to be clearer on when the same object can or can't be returned

bzbarsky has just created a new issue for =

https://github.com/whatwg/dom:

=3D=3D getElementsByTagName needs to be clearer on when the same object =

can or can't be returned =3D=3D
In particular, it looks to me like at call-time the =

`getElementsByTagName` call captures the HTML-ness of _root_'s node =

document at the time it's called.  So if that HTML-ness changes, a new
 object needs to be returned.

I put a testcase at =

http://web.mit.edu/bzbarsky/www/testcases/dom-queries/test_getElementsByTag=
Name_1.html
 which tries to exercise some of the edge-cases here.

See https://github.com/whatwg/dom/issues/90