JSFiddle - React, Tailwind, and code Playground

HTML

<div class="borded-textbox">Contrary to popular belief Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old.</div>

CSS

.borded-textbox
{
  width:200px;
  position:relative;
}

.endline.processed
{
  position:absolute;
  bottom:-1px;
  left:0px;
  height:10px;
  display:block;
  background-color:red;
  z-index:-1;
}

JavaScript

$(".borded-textbox").each(function(index, elem) {
  $(elem).html($(elem).html() + "<span class='endline'></span>");
  var endline=$(elem).find(".endline");
  endline.css('width', parseInt(endline.offset().left));  
  endline.addClass('processed');
});