Check If HTML Code Is Valid [JavaScript]

<textarea id="text" rows="5" cols="50">
<b>At</b> <a href="https://www.w3schools.com">w3schools.com</a> you'll learn how to make a <sup>website</sup>. 5 < 6!
</textarea>

<script>
   // https://stackoverflow.com/questions/10026626/check-if-html-snippet-is-valid-with-javascript
   // https://stackoverflow.com/questions/6093986/replace-all-strings-and-in-a-variable-with-lt-and-gt
   var text = document.getElementById('text');
   var textValue = text.value;
   function checkHTML(html) {
        var doc = document.createElement('div');
        doc.innerHTML = html;
        return doc.innerHTML === html;
    }
   var fixedTextValue = textValue.replace(/\</g,"&lt;").replace(/\>/g,"&gt;") // convert < and > to &lt; and &gt;
   // restore opening tags
   .replace(/&lt;b&gt;/g,"<b>").replace(/&lt;sup&gt;/g,"<sup>").replace(/&lt;sub&gt;/g,"<sub>")
   .replace(/&lt;a/g,"<a").replace(/\"&gt;/g,"\">")
   // restore closing tags
   .replace(/&lt;\/b&gt;/g,"</b>").replace(/&lt;\/sup&gt;/g,"</sup>").replace(/&lt;\/sub&gt;/g,"</sub>")
   .replace(/&lt;\/a&gt;/g,"<\/a>");
   alert(textValue + '\n\nvalid? ' + checkHTML(textValue));
   alert(fixedTextValue + '\n\nvalid? ' + checkHTML(fixedTextValue));
   // inputted text can contain html tags and < and/or > symbols
   // convert inputted text to fixed one before validating html
   // this way if we have < and/or > it will still be valid html as only < will be replaced by &lt; and > - by &gt;
   // the replacing will be done behind the scenes, will not be set to textarea value! Needed only to check HTML!
</script>