[css-backgrounds-3][editorial] Negative values are invalid for 3rd <shadow> length

[cdoublev]

... as currently specified in prose:

3rd <length [0,∞]>

Specifies the blur radius. Negative values are invalid.

... but the current syntax allows 1px 1px -1px to match:

<shadow> = <color>? && [<length>{2} <length [0,∞]>? <length>?] && inset?

... while this is invalid in mainstream browsers.

[SebastianZ]

The changed syntax is also what we already have in CSS Borders 4. So once the little syntax adjustment is done, I'm fine with it.

Sebastian

[SebastianZ]

Thanks @cdoublev!

[SebastianZ]

I've also added a note about this change to the Changes section.

Sebastian