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There is third part of value definition: [ center | [ left | right ] ] && [ center | [ top | bottom ] ] <length>? According to Css Values and Unit Module (http://dev.w3.org/csswg/css-values/#component-combinators) it should be interpreted as [ center | [ left | right ] ] && [ [ center | [ top | bottom ] ] <length>? ] Because 'Juxtaposition is stronger than the double ampersand'. It means that value 'left top 20cm' is valid. Since '&&' is re-orderable combinator the 'top 20cm left' value is valid too. But the description tells us that 'a third value always represents the Z position (or offset) and must be of type <length>' so 'top 20cm left' can't be valid. Please fix the description or add extra brackets in value definition like in following example: [ [ center | [ left | right ] ] && [ [ center | [ top | bottom ] ] ] <length>? Also it seems that brackets in '[left | right]' and '[top | bottom]' groups are redundant.
I changed it to: [ left | center | right | top | bottom | <percentage> | <length> ] | [ left | center | right | <percentage> | <length> ] [ top | center | bottom | <percentage> | <length> ] <length>? | [[ center | left | right ] && [ center | top | bottom ]] <length>? which I think is correct. I suspect it was copied from https://drafts.csswg.org/css-backgrounds-3/#position but without understanding the "edge plus delta" form that the final production is trying to address.