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http://www.w3.org/TR/filter-effects-1/#FilterUnitsAttribute says: "If filterUnits is equal to objectBoundingBox, then x, y, width, height represent fractions or percentages of the bounding box on the referencing element (see object bounding box units)." Now, if we take an example of a valid <length> for the x attribute, say "47em", how should that be interpreted in the case of filterUnits="objectBoundingBox"? I think the author's expection might be that 47em is kept as just that, but actually what happens is that it gets computed into user units and then taken as a fraction of the boundingbox. Therefore I'd like to ask that the behavior is clarified such that it becomes more obvious that this happens.
As discussed in the FX meeting (http://www.w3.org/2015/02/10-fx-irc) there is nothing that we need to change in Fitler Effects. CSS specifies this case sufficiently.