// partie pageArtivcle.php a changer
<div id="hellomark">

			<p class="text-justify">
			<?= $donnees['art_content']; ?>
			</p> <br>
			</div>
//sur une nouvelle page js

$(document).ready(function(){

  var str1 = $("#hellomark").text();
  var md1 = markdown.toHTML(str1);


  $("#hellomark").html(md1);
});

// puis les scripts


<script type="text/javascript" src="node_modules/jquery/dist/jquery.min.js"></script>
<script type="text/javascript" src="node_modules/markdown/lib/markdown.js"></script>
<script type="text/javascript" src="page/js/commentaire.js"></script>
<script type="text/javascript" src="page/js/markdown_test.js"></script>