Add support for exponential notation.

[daohoangson]

Related to dart-lang/csslib#123

[kevmoo]

@devoncarew ?

[devoncarew]

Thanks for the contribution!

I don't see the css validating using the validator at https://jigsaw.w3.org/css-validator.

1		Value Error : color Unknown dimension 1e2
1		Value Error : color Unknown dimension .5e1
1		Value Error : color Unknown dimension .5e-1
1		Value Error : color Invalid RGB function )

That site claims to be validating against CSS level 3.

[devoncarew]

I do see exponential notation in the railroad grammar for numeric tokens (https://drafts.csswg.org/css-syntax-3/#typedef-number-token), and can use it in css properties when editing things in devtools. I wonder - is the validator out of date? Incorrect?

[daohoangson]

Apparently all popular validators fail to parse this. It works in both Chrome and Safari though.

https://jsfiddle.net/36r8vpas/

[devoncarew]

Hmm, @leonsenft - do you have thoughts here?