The first version of the masking implementation for <mask> elements defined in SVG does not cache the used resources (see CachedSVGDocument::canReuse). The reason for this was the need to be notified when the SVG document loaded in order to check if a valid <mask> element can be find inside it and, if not, switch to using normal images. If the document was retrieved from cache, this notification would not fire (MaskImageOperation::notifyFinished). An improvement to this implementation would be to find a way around this.