2.1.13 [ECMA-262/5] Section 18.104.22.168, Number.prototype.toPrecision (precision)
An implementation is permitted to extend the behaviour of toPrecision for values of precision less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.
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The behavior of the toPrecision function is not extended to values of the precision property that are less than 1 or greater than 21.