JSFiddle - React, Tailwind, and code Playground

HTML

Non-editable text. Editable is below:
<div id="test" contenteditable="true">Hello, some <b>bold</b> and <i>italic and <b>bold</b></i> text</div>
<div id="caretPos"></div>

JavaScript

function getCaretCharacterOffsetWithin(element) {
    var caretOffset = 0;
    var doc = element.ownerDocument || element.document;
    var win = doc.defaultView || doc.parentWindow;
    var sel;
    if (typeof win.getSelection != "undefined") {
        sel = win.getSelection();
        if (sel.rangeCount > 0) {
            var range = win.getSelection().getRangeAt(0);
            var preCaretRange = range.cloneRange();
            preCaretRange.selectNodeContents(element);
            preCaretRange.setEnd(range.endContainer, range.endOffset);
            caretOffset = preCaretRange.toString().length;
        }
    } else if ( (sel = doc.selection) && sel.type != "Control") {
        var textRange = sel.createRange();
        var preCaretTextRange = doc.body.createTextRange();
        preCaretTextRange.moveToElementText(element);
        preCaretTextRange.setEndPoint("EndToEnd", textRange);
        caretOffset = preCaretTextRange.text.length;
    }
    return caretOffset;
}

function showCaretPos() {
    var el = document.getElementById("test");
    var caretPosEl = document.getElementById("caretPos");
    caretPosEl.innerHTML = "Caret position: " + getCaretCharacterOffsetWithin(el);
}

document.body.onkeyup = showCaretPos;
document.body.onmouseup = showCaretPos;