<my-custom></my-custom>
<script>
    class MyWebComponent extends HTMLElement {
        constructor() {
            super();
            this.innerHTML = `<h1>Hello World</h1>`;
            this.header = this.querySelector('h1');
        }

        changeTextColor(color) {   
            this.header.style.color = color;
        }

        changeText(text) {
            this.header.innerText = text;
        }
    }

    customElements.define('my-custom', MyWebComponent);
</script>