JSFiddle - React, Tailwind, and code Playground

HTML

<span id="startindex">Please select the ampersand and see it start. Than select the space after the ampersand and see the count difference(20-25 instead of 20-21)</span>
<div id="textBlock" style="width:300px; background: red; height:500px;">This is a cool test & stuff like that</div>

JavaScript

$(document).ready(function() {
	$('#textBlock').mouseup(function() {
		var selectionRange = window.getSelection();
		if (!selectionRange.isCollapsed) {
			selectedText = selectionRange.getRangeAt(0).toString();
		}

		document.getElementById('textBlock').setAttribute('contenteditable', true);
		document.execCommand('strikethrough', false);
		var startIndex = $('#textBlock').html().indexOf('<strike>');
		 $('#startindex').html('the startindex is: ' + startIndex);
		done();
	});
});

function done() {
	document.getElementById('textBlock').setAttribute('contenteditable', false);
	document.getSelection().removeAllRanges();
	removeStrikeFromElement($('#textBlock'));
}

function removeStrikeFromElement (element) {
	element.find('strike').each(function() {
		jQuery(this).replaceWith(removeStrikeFromElement(jQuery(this)));
	});
	return element.html();
}