JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id="removeHeadingsButton" value="Remove headings">
<div style="border: solid green 1px" contenteditable="true">
    <h1>Removing headings</h1>
    <p>Select some stuff in here and press the button above to remove headings</p>
    <h2>Subheading</h2>
    <p>Cheese on toast, barnacles.</p>
    <h3>Sub-subheading</h3>
    <p>Turnips, conquistadors.</p>
</div>

JavaScript

function nextNode(node) {
    if (node.hasChildNodes()) {
        return node.firstChild;
    } else {
        while (node && !node.nextSibling) {
            node = node.parentNode;
        }
        if (!node) {
            return null;
        }
        return node.nextSibling;
    }
}

function getRangeSelectedNodes(range, includePartiallySelectedContainers) {
    var node = range.startContainer;
    var endNode = range.endContainer;
    var rangeNodes = [];

    // Special case for a range that is contained within a single node
    if (node == endNode) {
        rangeNodes = [node];
    } else {
        // Iterate nodes until we hit the end container
        while (node && node != endNode) {
            rangeNodes.push( node = nextNode(node) );
        }
    
        // Add partially selected nodes at the start of the range
        node = range.startContainer;
        while (node && node != range.commonAncestorContainer) {
            rangeNodes.unshift(node);
            node = node.parentNode;
        }
    }

    // Add ancestors of the range container, if required
    if (includePartiallySelectedContainers) {
        node = range.commonAncestorContainer;
        while (node) {
            rangeNodes.push(node);
            node = node.parentNode;
        }
    }

    return rangeNodes;
}

function getSelectedNodes() {
    var nodes = [];
    if (window.getSelection) {
        var sel = window.getSelection();
        for (var i = 0, len = sel.rangeCount; i < len; ++i) {
            nodes.push.apply(nodes, getRangeSelectedNodes(sel.getRangeAt(i), true));
        }
    }
    return nodes;
}

function replaceWithOwnChildren(el) {
    var parent = el.parentNode;
    while (el.hasChildNodes()) {
        parent.insertBefore(el.firstChild, el);
    }
    parent.removeChild(el);
}

function removeSelectedElements(tagNames) {
    var tagNamesArray = tagNames.toLowerCase().split(",");
    getSelectedNodes().forEach(function(node) {
        if (node.nodeType == 1...