[css-typed-om] CSSPositionValues should be created as a function of what the property accepts

[dbaron]

CSSPositionValue normalization begins with:

If the provided value matches the <position> production, then a CSSPositionValue is constructed with x and y components determined via the following process.

This seems to imply that CSSPositionValues are created as a function of the syntax that happens to be present (e.g., for 'border-width: 3px 4px`) rather than as a function of the syntax that the property accepts. It should be specified like the other value types, in terms of the syntax that the property accepts.