写个函数返回一个页面里共使用了多少种HTML tag

运用到ES6的Set数据类型：

let tags = [...document.getElementsByTagName('*')].map((v)=>{
  return v.tagName
})
tags = [...new Set(tags)]

https://stackoverflow.com/questions/15763358/difference-between-htmlcollection-nodelists-and-arrays-of-objects