JSFiddle - React, Tailwind, and code Playground

HTML

<shadow-はやと>
</shadow-はやと>

JavaScript

var XFooProto = Object.create(HTMLElement.prototype);

XFooProto.createdCallback = function() {
  this.innerHTML = "<b>I'm a shadow-はやと!</b>";
};

var XFoo = document.registerElement('shadow-はやと', {prototype: XFooProto});