[Aetae]

var text = document.getElementById('post_message_475649').firstChild;
var range = document.createRange();
range.selectNode(text);
var rects = range.getClientRects(); //большой прямоугольник из маленьких состоит
[].forEach.call(rects, ({top, left, width, height}) => document.body.insertAdjacentHTML(
  'beforeend', 
  `<div style="
    top: ${top}px;
    left: ${left}px;
    width: ${width}px;
    height: ${height}px;
    position: fixed;
    outline:1px dotted rgba(0,0,255,0.5);
  "></div>`
));
range.detach();