An mi element represents a symbolic name or arbitrary text that should be rendered as an identifier. Identifiers can include variables, function names, and symbolic constants. MathML Fundamentals
<math>
<mi>xx</mi>
<mo>+</mo>
<mi>x</mi>
<mo>+</mo>
<mi mathvariant="bold">x</mi>
<mo>+</mo>
<mi mathcolor="#ff0000">y</mi>
<mo>+</mo>
<mi mathcolor="blue">z</mi>
<mo>+</mo>
<mi mathbackground="#ff8844">w</mi>
<mo>+</mo>
<mi mathbackground="blue">x</mi>
</math>