Handling very long fractions (600px width used for image). Second line should be centered.
<math xmlns="http://www.w3.org/1998/Math/MathML" indentalign="center" display="block"> <mfrac> <mrow> <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> <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> <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> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> <mo>+</mo> <mi>x</mi> </mrow> <mrow> <mi>y</mi> <mo>+</mo> <mi>y</mi> <mo>+</mo> <mi>y</mi> </mrow> </mfrac> </math>