Applies outline and background-color in order to more readily understand layouts.

outliner-snippet.js

if (!document.querySelector("#debugStyles")) {
  const style = document.createElement("style");
  style.setAttribute("id", "debugStyles");
  style.innerHTML = `
        .debug-boxes * {
            outline: 1px solid blue;
            background-color: rgba(0,255,255,.05);
        }
    `;
  document.head.appendChild(style);
  console.info("Debug CSS added to document head");
}
$0.classList.add("debug-boxes");

Paste as a snippet in your devtools and select the DOM node subtree you want boxed out and run from the command palette.