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6.6.2 Serializing CSS Values does not list if there should be white-spaces after the semi-colons or not in the serialized output. For example, 'margin: 20px 20px 20px 20px;' is output serialized as 'margin: 20px; ' in Webkit while it is output serialized as 'margin: 20px' in Firefox. http://dev.w3.org/csswg/cssom/#serializing-css-values
The part of the spec you linked to doesn't serialize the semicolon. It is what getPropertyValue returns. If you mean .cssText on CSSStyleDeclaration, this is specced to include the semicolon but no trailing space after the last declaration. http://dev.w3.org/csswg/cssom/#serialize-a-css-declaration-block