mpadded looks like the right MathML rendering for \raisebox (currently it's rendered like \text). As @ronkok writes:

I think that one can get effective presentation MathML for \raisebox with code something like:

const node = new mathMLTree.MathNode(
        "mpadded", [buildGroup(group.value.body, options)]);
node.setAttribute("voffset", group.value.dy.value);

I have not tested that code, but I have put the HTML below into Firefox

<math xmlns="http://www.w3.org/198/Math/MathML">
    <mi>a</mi><mpadded voffset="0.5em">b</mpadded><mi>c</mi>
</math>

... and it comes out looking like this: