JSFiddle - React, Tailwind, and code Playground

HTML

<a id="sample_id" onblur="color_overlength_func('sample_id', 20)" contenteditable="true">Some Text</a>

JavaScript

function color_overlength_func(textfield_id, max_length) {
  let text_temp = document.getElementById(textfield_id).innerHTML;
  if (text_temp.length >= max_length) {
    let text_OK = text_temp.substr(0, max_length);
    let text_to_long = text_temp.substr(max_length);
    document.getElementById(textfield_id).innerHTML = "" + text_OK + "<em style='color:red;'>" + text_to_long + "</em>";
  }
}