Chapter 4  Syntax and basic data types  4.3.3 Percentages
Go to the relevant page of the specification.
 c62percent000 (ahem, image)

Percentage Units
 numbersunits016 (invalid)

Parsing invalid percentage values  decimal value with multiple decimals
Percentage assignments with more than one decimal are invalid.  numbersunits017

Parsing of percentage values  0%
Percentage of 0% is valid and calculates to 0.