JSFiddle - React, Tailwind, and code Playground

HTML

<div><p>A <em>partir</em> d'ici ce n'est <strong>que</strong> du texte sélectionné</p></div>
<p class="result"></p>

JavaScript

document.body.addEventListener("mouseup", function(e) {
    var p = document.getElementsByClassName('result'),
        selection = window.getSelection(),
        str = selection.focusNode.data.toString();
    console.log(selection);
    p[0].innerHTML = str.substring(selection.anchorOffset, selection.focusOffset);
});