18.3.18 length and 19.573 svg:width - percentage and negative length

Datatype of svg:width is length. In ODF 18.3.18 length is defined as "A (positive or negative) length, consisting of magnitude and unit, in conformance with the Units of Measure defined in §5.9.13 of [XSL]".

In SVG: http://www.w3.org/TR/SVG/types.html#DataTypeLength a destinction is made between style sheets and presentation attributes. The difference is support for percentage not found in [XSL]. "Note that the non-property <length> definition also allows a percentage unit identifier. The meaning of a percentage length value depends on the attribute for which the percentage length value has been specified. Two common cases are: (a) when a percentage length value represents a percentage of the viewport width or height (refer to the section that discusses units in general), and (b) when a percentage length value represents a percentage of the bounding box width or height on a given object (refer to the section that describes object bounding box units)."

Furthermore [SVG] interface defines for most length values positive values (unsigned short)
http://www.w3.org/TR/SVG/types.html#InterfaceSVGLength

For what reasons does 18.3.18 permit negative length and omits the %?