JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <p>Testing paragraph</p>
    <p>Testing paragraph</p>
</div>
<div>
    <p>Testing <b>lol</b> paragraph</p>
    <p>Testing paragraph</p>
</div>

CSS

body * {
  outline: 10px pink inset;
  outline-offset: -10px;
  padding: 10px;
  opacity: .9;
  margin: 20px;
}

JavaScript

var last;
window.addEventListener("mousemove", function(e) {
  if(last) {
  	last.style.background = ''; // empty is enough to restore previous value
  }
	var elem = e.target;
  
  if(elem === document.body || elem === document.documentElement) {
  	return;
  }
  var bb = elem.getBoundingClientRect();
  var xr = e.pageX - bb.left; // x relative to elem
  var yr = e.pageY - bb.top; // y relative to elem
  var ew = 10; // edge width
  
  if(
  	   xr <= ew
  	|| xr >= bb.width - ew
    || yr <= ew
    || yr >= bb.height - ew
  ){
    elem.style.background = 'red';
    last = elem;
  }
});