Get a list of custom elements used on a page

detect_custom_elements.js

// Highlights all custom elements on the page.
// 7/31/2016: updated to work with both shadow dom v0 and v1.
// To create a bookmarklet, use http://ted.mielczarek.org/code/mozilla/bookmarklet.html

var allCustomElements = [];

function isCustomElement(el) {
  const isAttr = el.getAttribute('is');
  // Check for <super-button> and <button is="super-button">.
  return el.localName.includes('-') || isAttr && isAttr.includes('-');
}

function findAllCustomElements(nodes) {
  for (let i = 0, el; el = nodes[i]; ++i) {
    if (isCustomElement(el)) {
      allCustomElements.push(el);
    }
    // If the element has shadow DOM, dig deeper.
    if (el.shadowRoot) {
      findAllCustomElements(el.shadowRoot.querySelectorAll('*'));
    }
  }
}

findAllCustomElements(document.querySelectorAll('*'));

let tagNames = [...new Set(allCustomElements.map(el => el.nodeName.toLowerCase()))];

if (tagNames.length > 0) {
  console.log("Unique tag names:", tagNames);
  console.log("All custom elements:", allCustomElements);
}
else {
  console.log("No custom elements found");
}