hello-world.html

<script>
class HelloWorld extends HTMLElement{
    constructor(){
        //always first
        super()
        const shadow = this.attachShadow({mode:'open'});
      shadow.innerHTML = 
`
<style>
.content { 
    font-weight: bold;
    background: #F5F5F5;
}
</style>
<div class="content">Hello world</div>
`
    }
}

customElements.define('hello-world', HelloWorld);
</script>