2.1.8 [CSS-Level2-2009] Section 4.3.2, Lengths

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V0006:

The specification states: 

 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.

Quirks Mode and IE7 Mode (All Versions)

Lengths can be specified in scientific notation; for example, ‘5e2px’ instead of ‘500px’.

Quirks Mode (All Versions)

Spaces are allowed between <number> and the unit identifier.