'use strict';

console.clear();

let body = document.getElementsByTagName('body')[0];

function findCustomTags(node) {

  	if (node instanceof HTMLUnknownElement || (''+node.tagName).indexOf('-') !== -1) {
  		return (''+node.tagName).toLowerCase();
    }

    let custom_tags = [];

    // recurse into children
  	for (let i = 0; i < node.childNodes.length; i++) {
    	let new_tags = findCustomTags(node.childNodes[i]);
      custom_tags = custom_tags.concat(new_tags);
  	};

    return custom_tags;
}


console.log(findCustomTags(body));