Do I understand correctly that the old parser allows quoted attributes and multiple strings in attribute values, but does not allow multiple attributes without whitespace in between or multiple strings in attribute values _with_ whitespace in between?
So
<t a="foo" "bar">
would have two attributes "a" and "bar",
and
<t a="foo""bar">
would have one attribute "a"?
Neither of these is valid HTML, so there is no particular reason to change the interpretation of either.
<t a="foo"b="bar">
does not have two quoted strings after each other, so there is no need to consider it for concatentation, and it can be interpreted as two attributes "a" and "b".
Is there an example of something which is valid HTML _and_ which would (for good reason) be interpreted as something else by the old parser?