JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test"> Regualar text <b>Strong text</b> </div>

CSS

b { color:red; }
strong { color:green; }

JavaScript

function toArray( arrayLike ) {
    return [].slice.call( arrayLike );
}

var root, elems;

root = document.getElementById( 'test' );
elems = root.getElementsByTagName( 'b' );

toArray( elems ).forEach( function ( elem ) {
    var newElem = document.createElement( 'strong' );
    newElem.textContent = elem.textContent;
    elem.parentNode.replaceChild( newElem, elem );    
});