Fix bug #79701: getElementById does not correctly work with duplicate definitions

This is a long standing bug: IDs aren't properly tracked causing either
outdated or plain incorrect resu

Fix bug #79701: getElementById does not correctly work with duplicate definitions

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outdated or plain incorrect results from getElementById.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.

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