class InnerComponent extends HTMLElement {
    constructor() {
        super();

        const shadowRoot = this.attachShadow({mode: "open"});

        shadowRoot.innerHTML = `
            <style>
                :host {
                    display: inline;
                }
            </style>
            
            <div>Inner Text</div>
        `;
    }
}

class OuterComponent extends HTMLElement {
    constructor() {
        super();

        const shadowRoot = this.attachShadow({mode: "open"});

        shadowRoot.innerHTML = `
            <style>
                :host {
                    display: inline;
                }
            </style>
            
            <div>Outer Text</div>
            <inner-component></inner-component>
        `;
    }
}

window.customElements.define("inner-component", InnerComponent);
window.customElements.define("outer-component", OuterComponent);