Currently, we return StyleParseError::UnspecifiedError  for invalid values in many places. However, the actual error is that we're parsing invalid values. Xidorn suggests maybe we could use something like PropertyDeclarationParseError::InvalidValue instead.  http://searchfox.org/mozilla-central/rev/d840ebd5858a61dbc1622487c1fab74ecf235e03/servo/components/style/properties/longhand/font.mako.rs#1563
Hi Josh, since you're the author of these changes, we'd like to have your feedback before going any further.
UnspecifiedError is intended as a placeholder for code that could benefit from returning more specific errors indicating what actually went wrong. I do not think that returning StyleParseError::PropertyValue(PropertyValueParseError::InvalidValue) will be a meaningful improvement at the moment, however.
Lower the priority and un-assign myself according to comment 2.