Get names of all elements on a web page with JS

This tip is originally from Lea Verou.

Get the names of all elements on the page:

let names = [...new Set($$("*").map(e => e.nodeName.toLowerCase()))];

Get all names in the body:

let names = [...new Set($$("body *").map(e => e.nodeName.toLowerCase()))];

Get all class names used:

let classes = [...new Set($$("[class]").flatMap(e => [...e.classList]))];