Bug 123348

Currently, certain invalid MathML documents (such as a MathML document containing arbitrary HTML markup) is rendered improperly. According to the MathML specification, we should be providing better error information. From http://www.w3.org/TR/MathML3/chapter6.html), Section 6.4 Combining MathML and Other Formats: "In the standard schema, elements from other namespaces are not allowed, but attributes from other namespaces are permitted. MathML processors that encounter unknown XML markup should behave as follows: An attribute from a non-MathML namespace should be silently ignored. An element from a non-MathML namespace should be treated as an error, except in an annotation-xml element. If the element is a child of a presentation element, it should be handled as described in Section 3.3.5 Error Message <merror>. If the element is a child of a content element, it should be handled as described in Section 4.2.9 Error Markup <cerror>. For example, if the second child of an mfrac element is an unknown element, the fraction should be rendered with a denominator that indicates the error."