In Section 3, Full-Text Selections, the last sentence of the third paragraph reads: The RangeExpr is evaluated, as if it were an argument to a function with an expected type xs:double; it must be between 0.0 and 1000.0 inclusive.

That sentence does not specify what should happen if a weight value is provided that is outside of the range, nor does anything else I can find in the spec.  If weight were always a literal, then perhaps we could just make it a parse error.  But weight is a RangeExpr, which can also be things like variables. 

Reasonable choices for "what happens" might be:

* Raise a specific error (to be determined). 

* Turn all values greater than 1000.0 into 1000.0 and values less than 0.0 into 0.0. 

Unreasonable choices might be to return nothing at all (bad because implementations might assign a meaning to out-of-range values and that would make it more difficult for us to do so in the future) or to compute the result as the specified value modulo 1000.0 (bad because 1000.1 would become 0.1). 

On a related note, I observe that the statement in Section 3 is absolute, while Section 5.2.15, "Weights" says: An implementation may constrain the range of valid weights to non-negative values.

From the text in Section 3, I don't see how implementations need to anything at all to "constrain the range".  Considering that Section 5.2 is Optional Features, it's unclear to me what the intent of 5.2.15 really is.