Many properties only accept positive values. Negative values are illegal, and are parse errors. This is currently only defined in prose for the relevant property. Examples: width, height, line-height.
In contrast, other properties have logical bounds, but values outside these bounds are parsed as normal and only clamped at used value time. Examples: opacity, numeric parameters in color functions.
I'm sure it would be useful for both authors and implementers if the property value syntax explicitly indicated whether negative values would be a parse error or not. That requires new value types in CSS Values & Units.
I'm not sure if there are any other common "illegal value" restrictions currently defined in prose, but it's worth looking around to see if there are, and if they can also be standardized into the value definition syntax.
frivoal, SebastianZ, kizu and roger-russel
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