You currently have to cast renderers to RenderMathMLBlock before you can do further type checking. We should move all those isRenderMathMLFoo() functions to RenderObject.
Created attachment 216508 [details] Patch
Created attachment 216509 [details] Patch Oops, first patch broke accessibility/mathml-elements.html
Committed r159009: <http://trac.webkit.org/changeset/159009>