JSFiddle - React, Tailwind, and code Playground

HTML

<p>
    Paragraph element
    <span>inner span element</span>
    <a href="#">inner anchor element</a>
</p>

<p>
    Paragraph element
    <span>inner span element</span>
    <a href="#">inner anchor element</a>
</p>

<span>Span element</span>

<a href="#">Anchor element</a>

<div id="mydiv">
    Div element
    <a href="#">DIV anchor element 1</a>
    <a href="#">DIV anchor element 2</a>
    <a href="#">DIV anchor element 3</a>
    
    <p>
        Paragraph element
        <span>inner span element</span>
        <a href="#">inner anchor element</a>
    </p>
</div>

<hr />

<p>
    Number of anchors and paragraphs in document: <span id="counter1"></span>
</p>

<p>
    Number of anchors and paragraphs inside <strong>div#mydiv</strong>: <span id="counter2"></span>
</p>

JavaScript

Node.prototype.getElementsByTagNames = function (tags) {
    var elements = [];
    
    for (var i = 0, n = tags.length; i < n; i++)
    {
        elements = elements.concat(Array.prototype.slice.call(this.getElementsByTagName(tags[i])));
    }

    return elements;
};

var elems, mydiv = document.getElementById('mydiv');

elems = document.getElementsByTagNames(['p', 'a']);

document.getElementById('counter1').innerHTML = elems.length;

elems = mydiv.getElementsByTagNames(['p', 'a']);

document.getElementById('counter2').innerHTML = elems.length;