feat: support for custom and slot elements (#2581)

tests/reftests/webcomponents/autonomous-custom-element.js

@@ -0,0 +1,45 @@
+class AutonomousCustomElement extends HTMLElement {
+    constructor() {
+        super();
+
+        const shadow = this.attachShadow({mode: 'open'});
+        const wrapper = document.createElement('span');
+        wrapper.setAttribute('class', 'wrapper');
+
+        const info = document.createElement('span');
+        info.setAttribute('class', 'info');
+
+        info.textContent = this.getAttribute('text');
+
+        const img = document.createElement('img');
+        img.src = this.getAttribute('img');
+
+        // Create some CSS to apply to the shadow dom
+        const style = document.createElement('style');
+
+        style.textContent = `
+      .wrapper {
+        position: relative;
+      }
+      .info {
+        font-size: 0.8rem;
+        width: 200px;
+        display: inline-block;
+        border: 1px solid black;
+        padding: 10px;
+        background: white;
+        border-radius: 10px;
+      }
+      img {
+        width: 100px;
+      }
+    `;
+
+        shadow.appendChild(style);
+        shadow.appendChild(wrapper);
+        wrapper.appendChild(img);
+        wrapper.appendChild(info);
+    }
+}
+
+customElements.define('autonomous-custom-element', AutonomousCustomElement);