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>