The template element in HTML 5 is special in the sense that it does not
add its contents into the DOM tree, but instead keeps them in a separate
shadow DOM document fragment. Interacting with the DOM tree cannot touch
the elements in the document fragment.
Closes GH-14906.
This is a long standing bug: IDs aren't properly tracked causing either
outdated or plain incorrect results from getElementById.
This PR implements a pragmatic solution in which we still try to use the
ID lookup table to a degree, but only as a performance boost not as a
"single source of truth". Full details are explained in the
getElementById code.
Closes GH-14349.
