testing mathbackground color attribute
<math xmlns="http://www.w3.org/1998/Math/MathML"> <mstyle mathbackground="yellow"> <mo>(</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>)</mo> </mstyle> </math>