JSFiddle - React, Tailwind, and code Playground
HTML
<math-item>
<math-source>Foo A</math-source>
<math-source>Foo B</math-source>
</math-item>
CSS
math-item math-source {
display: none;
}
JavaScript
var item, root, content;
item = document.querySelector('math-item');
root = item.createShadowRoot();
root.innerHTML = '<style>::content math-source:nth-child(2) {display: inline-block;}</style><content></content>';