<pre class="de1">void function () {
const canvas = document.createElement('canvas')

canvas.width = document.documentElement.offsetWidth
canvas.height = document.documentElement.offsetHeight

canvas.style.position = 'absolute'
canvas.style.left = '0'
canvas.style.right = '0'
canvas.style.top = '0'
canvas.style.bottom = '0'
canvas.style.zIndex = '99999'

document.body.appendChild(canvas)

const ctx = canvas.getContext('2d')
draw(ctx,getAllRects())

function draw (ctx,rects) {
let i = 0
ctx.strokeStyle = 'red'
window.requestAnimationFrame(_draw)

function _draw () {
  let {x,y,width,height} = rects[i++]
  ctx.strokeRect(x,height)
  if (i < rects.length) {
    window.requestAnimationFrame(_draw)
  } else {
    console.log('%cDONE','background-color: green; color: white; padding: 0.3em 0.5em;')
  }
}

}

function getAllRects () {
const allElements = document.querySelectorAll('*')
const rects = []
const {x: htmlX,y: htmlY} = document.documentElement.getBoundingClientRect()
allElements.forEach(element => {
const eachElRects = Array.from(element.getClientRects()).filter(rect => {
return rect.width || rect.height
}).map(rect => {
return {
x: rect.x - htmlX, y: rect.y - htmlY, width: rect.width, height: rect.height
}
})
rects.push(...eachElRects)
})
return rects
}
}()