js

getBoundingClientRect()

html

<div id="highlight"></div>
<p>
  This example positions a "highlight" rectangle behind the contents of a range.
  The range's content <em>starts here</em> and continues on until it
  <em>ends here</em>. The bounding client rectangle contains everything selected
  in the range.
</p>

css

#highlight {
  background: yellow;
  position: absolute;
  z-index: -1;
}

p {
  width: 200px;
}

js

const range = document.createRange();
range.setStartBefore(document.getElementsByTagName("em").item(0));
range.setEndAfter(document.getElementsByTagName("em").item(1));

const clientRect = range.getBoundingClientRect();
const highlight = document.getElementById("highlight");
highlight.style.left = `${clientRect.x}px`;
highlight.style.top = `${clientRect.y}px`;
highlight.style.width = `${clientRect.width}px`;
highlight.style.height = `${clientRect.height}px`;