SVG output: unkown mathbackground, unkown color problem

The HTML-CSS output ignores these unkown properties but the SVG output interprets them (badly)

Example:

    <math xmlns="http://www.w3.org/1998/Math/MathML">
      <mn mathbackground="quagmire" style="background-color:quagmire;">2</mn>
      <mo color="potato">+</mo>
      <mn>5</mn>
    </math>