Why can't you create Custom Elements in content scripts?

Question

I attempted to create a custom element in a Chrome extension content script but customElements.define is null.

customElements.define('customElement', class extends HTMLElement {
    constructor() {
        super();
    }
    ...
});

So apparently Chrome doesn't want content scripts to create custom elements. But why? Is it a security risk?

I can't seem to find anything in Chrome's extension guide that says it's not allowed.

Answer 1

As of now custom element can be used in chrome extensions UI. In Popup ui, option page ui and in the content script as well But it requires a polyfill which is this. https://github.com/GoogleChromeLabs/ProjectVisBug - this is the one big custom element in the chrome extension.