See http://www.w3.org/TR/MathML/chapter3.html#id.3.4.4.2 http://www.w3.org/TR/MathML/chapter3.html#id.3.4.5.2 To determine the default value, we first need the operator dictionary.
This is actually implemented by bug 155756
Fixed by https://trac.webkit.org/changeset/203074