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In 4.2, there is an example that reads "CSS3 module: Syntax [CSS3-SYNTAX] defines an extension mechanism which uses a not valid start character for identifiers in CSS, so it is guaranteed never to be used by any current or future level of CSS. CSS-conforming parsers will skip rules that contain identifiers with such a character. Therefore it is not authorized to redefine the properties defined in the specification." -- this has been changed in CSS2.1. The properties are not valid, but the working group guarentees to never start a standard identifier with a hyphen, which has the same result.
The WG agreed to proceed with this editorial modification.
setting version to LC in case of future use