JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <x-tag1>1
        <section>
            <x-tag1-5>1.5</x-tag1-5>
        </section>
    </x-tag1>
    <x-tag2>2</x-tag2>
</div>

<p id="result">init</p>

JavaScript

$(function(){
    
    var results = $("div").find("*").filter(function(){
        return /^x\-/i.test(this.nodeName);
    });
    
    $("#result").text("found elements: " + results.length);
});