Example code of how a Math preview could be registered

RegisterMathP

// do this on the server
document.querySelectorAll('.mwe-math-element img').forEach((node) => {
    node.dataset.title = 'math-unique-identifier';
});
const previewType = 'math'
mw.popups.register(previewType, '.mwe-math-element img', {
    fetchPreviewForTitle: function ( title, el ) {
        console.log('fetch', title, el);
        const model = {
            title: 'Math',
			url: `#infoboxLink-${encodeURIComponent(title)}`,
			extract: 'Hello world',
			type: previewType,
            thumbnail: undefined,
			pageId: -1
		};

		return $.Deferred().resolve( model )
    }
})