common/src/web/webComponents.html

<style>
    custom-checkbox-element {
        display: block;
        width: 20px;
        height: 20px;
    }
</style>
<custom-checkbox-element></custom-checkbox-element>
<script>
    customElements.define('custom-checkbox-element',
        class extends HTMLElement {
            constructor() {
                super();
                this.attachShadow({ mode: 'open' }).innerHTML = `
                                <div><input type="checkbox"/>
                                <a href="simpleTest.html">Example Link</a>
                                </div>
                            `;
            }
        });
</script>