When the <msqrt> element is initially empty, nothing is drawn. When the first renderer child is added, an anonymous <mo> radical sign is inserted. In order to be consistent, this radical sign should be removed when we remove all the child elements of <msqrt>. However, this caused bug 133899 so for now I've kept the anonymous <mo> radical sign and marked mathml/roots-removeChild.html as failing.
Perhaps a better method would be to create the anonymous <mo> radical sign immediately from the beginning, and not to wait that a first child is inserted.
This issue is fixed on Alex's MathMLLayout branch, after the RenderMathMLRoot refactoring.
This test passes after http://trac.webkit.org/changeset/202168 and the TestExpectation has been updated.