<!DOCTYPE HTML> <html> <head> <script> function tweak() { // div with style "border: 10px solid green" var shadowDiv = document.createElement("div"); shadowDiv.style.border = "10px solid green"; var slot = document.createElement("slot"); shadowDiv.appendChild(slot); var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'}); shadowRoot.appendChild(shadowDiv); } </script> </head> <body onload="tweak()"> <div id="outer">Hello</div> </body> </html>