The terms first/second operand and left/right-hand operands makes it a bit unclear what is meant. This happens in at least two situations.

1.
19.8.7.5 Streamability of Simple Mapping Expressions (http://www.w3.org/TR/xslt-30/#streamability-of-simple-mapping-expressions)

Here the text explains to rewrite the expression to make the mapping operator left-associative. Then the text mentions 

"The posture and sweep of the expression are the posture and sweep of the right-hand operand, assessed with a context posture and type set to the posture and type of the first operand."

The last term here, "the first operand" should apply to the left-hand operand of the binary left-associative operator, but when reading it I was stumped whether it applied to the actual first operand of the whole expression (a in a!b!c).

2.
19.8.7.6 Streamability of Path Expressions (http://www.w3.org/TR/xslt-30/#streamability-of-path-expressions)

The same rewriting takes place here, making the path operator left-associative. Then, the text mentions:

"The posture of the expression is the posture of the right-hand operand, assessed with a context posture and type set to the posture and type of the first operand."

Again, the last term here is ambiguous for the same reasons as with the mapping operator. It's probably better to use right-hand and left-hand to make clear(er) what is meant.