Consider also Postel's Prescription:[10]
“Be liberal in what you accept, and conservative in what you
send”. Postel was speaking of network service programs, but the
underlying idea is more general. Well-designed programs cooperate
with other programs by making as much sense as they can from
ill-formed inputs; they either fail noisily or pass strictly clean and
correct data to the next program in the chain.
However, heed also this warning:
The original HTML documents recommended “be generous in
what you accept”, and it has bedeviled us ever since because
each browser accepts a different superset of the specifications. It
is the specifications that should be generous,
not their interpretation.
--Doug McIlroy
McIlroy adjures us to design for
generosity rather than compensating for inadequate standards with
permissive implementations. Otherwise, as he rightly points
out, it's all too easy to end up in tag soup.
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