There's probably a better way of doing this, though, I am stubborn and refuse to find out, this is good enough. Probably some bugs I have yet to encounter due to my arguably, nonexistent testing.

cursed-mathml-macro.rs

macro_rules! mathml_impl {
    () => {
        ""
    };

    ($ident:ident $($tt:tt)*) => {
        ::core::concat!(
            "<mi>",
            ::core::stringify!($ident),
            "</mi>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    // This is a workaround for issues relating to parsing unary negations.
    (~ $term:tt $($tt:tt)*) => {
        ::core::concat!(
            "<mrow>",
            "<mo>-</mo>",
            $crate::macros::mathml_impl!($term),
            "</mrow>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ($num:literal $($tt:tt)*) => {
        ::core::concat!(
            "<mn>",
            ::core::stringify!($num),
            "</mn>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };


    (* $($tt:tt)*) => {
        ::core::concat!(
            "<mo>&times;</mo>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ($term:tt _^ $sub:tt $sup:tt $($tt:tt)*) => {
        ::core::concat!(
            "<msubsup>",
            $crate::macros::mathml_impl!($term),
            $crate::macros::mathml_impl!($sub),
            $crate::macros::mathml_impl!($sup),
            "</msubsup>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ($term:tt ^ $sup:tt $($tt:tt)*) => {
        ::core::concat!(
            "<msup>",
            $crate::macros::mathml_impl!($term),
            $crate::macros::mathml_impl!($sup),
            "</msup>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ($term:tt _ $sub:tt $($tt:tt)*) => {
        ::core::concat!(
            "<msub>",
            $crate::macros::mathml_impl!($term),
            $crate::macros::mathml_impl!($sub),
            "</msub>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    (( $($inner:tt)* ) $($tt:tt)*) => {
        ::core::concat!(
            "<mrow>",
            "<mo>", "(", "</mo>",
            $crate::macros::mathml_impl!($($inner)*),
            "<mo>", ")", "</mo>",
            "</mrow>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ([ $($inner:tt)* ] $($tt:tt)*) => {
        ::core::concat!(
            "<mrow>",
            $crate::macros::mathml_impl!($($inner)*),
            "</mrow>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };

    ($op:tt $($tt:tt)*) => {
        ::core::concat!(
            "<mo>",
            ::core::stringify!($op),
            "</mo>",
            $crate::macros::mathml_impl!($($tt)*)
        )
    };
}

macro_rules! mathml {
    ($($tt:tt)*) => {
        ::core::concat!(
            r#"<math xmlns="http://www.w3.org/1998/Math/MathML">"#,
            $crate::macros::mathml_impl!($($tt)*),
            "</math>"
        )
    };
}

pub(crate) use mathml;
pub(crate) use mathml_impl;