JSFiddle - React, Tailwind, and code Playground

HTML

<div id="footer"><flow:fileref> This is a  flow:fileref tag  </flow:fileref></div>

<br>
<br>

elements finded with :eq(0) : <span id="result1">0</span>
<br>
elements finded without :eq(0) : <span id="result2">0</span>

JavaScript

$("#result1").text( $("div#footer>flow\\:fileref:eq(0)").length );
$("#result2").text( $("div#footer>flow\\:fileref").length );