Lengths refer to horizontal or vertical measurements.
The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.
Some properties allow negative length values, but this may complicate the formatting model and there may be implementation-specific limits. If a negative length value cannot be supported, it should be converted to the nearest value that can be supported.
If a negative length value is set on a property that does not allow negative length values, the declaration is ignored.
Complete description of lengths.