The SVG conditional processing attributes (requiredFeatures, requiredExtensions, and systemLanguage) are rarely used.  Currently nearly every SVG element creates SVGStringList objects for these three attributes in WebCore::SVGTests, at a cost of 48 bytes each SVGStringList plus the three pointers.  We can move these to SVGElementRareData to save 72 bytes per SVG element in this common case, which for SVG documents with a large number of graphical elements can add up.