2.1.3 [HTML] Section 6.6, Lengths
MultiLength: The value ( %MultiLength; in the DTD) may be a %Length; or a relative length. A relative length has the form "i*", where "i" is an integer. When allotting space among elements competing for that space, user agents allot pixel and percentage lengths first, then divide up remaining available space among relative lengths.
All Document Modes (All Versions)
Relative length values for attribute lengths of type MultiLength are not supported.