The old (very incomplete) definition of CSSStyleValue.parse() said that things returned null if they didn't parse. I've rewritten it in a proper algorithmic style, and identified three possible error locations. Which should throw, and which should return null? (Currently, the spec throws for all of them.)

1. Property name isn't an ident.
2. Property name is an ident, but not recognized as a CSS property.
3. Valid property name, but property value doesn't match the grammar of that property.