informal survey - on spec philosophy

It has been stated to me that, at least for "open web platform standards",
the following statement is true and is shared by the majority:

"if it isn't written in the spec, it isn't allowed by the spec"

I happen to disagree with the truth of this, based on my personal
experience both with spec writing and with implementation/use of specs, but
I would be curious to see who agrees with this idea or not.

The case in point is an instance of a possible ambiguity in a spec because
a particular assumption/convention is not documented; i.e., an assumption
that something isn't allowed even though it isn't explicitly disallowed.
While I agree it is, in general, impossible (or at least impractical) to
document all disallowances, I do believe it is important to document
important disallowances, particular when there are concerns raised about
spec ambiguity.

Regards,
Glenn