[whatwg] Update to the Adoption Agency Algorithm

[Gervase Markham]

Lachlan Hunt wrote:
> Errors caused by the result of duplicate IDs either in the markup or
> indirectly as a result of badly nested elements can be fixed by a quick
> visit to the validator (or other conformance tool) or by making use of
> any or all of those tools I mentioned above.

It's much harder to spot them when they are dynamically generated by
e.g. a cloneNode operation. You can't submit your browser's DOM tree to
the validator...

Gerv