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Proposal from Chris Marrin on www-style (http://lists.w3.org/Archives/Public/public-fx/2011OctDec/0146.html) "If either the vertex or fragment cannot be loaded, a shader fails to compile, or the shader program fails to link, then the custom() function becomes a pass through filter".
Authors can specify fall backs with the new @filter rule (for the case that one shader doesn't load, take the next one).