JSFiddle - React, Tailwind, and code Playground

HTML

<textarea>some text with wrong wrd</textarea>

CSS

.jquery-spelling-incorrect
{
    color:red;
    text-decoration:underline;
}

JavaScript

var dictionary = ['some', 'text', 'with', 'wrong', 'words'];

jQuery('textarea').hide().before('<div class="jquery-spelling-container" contenteditable="true"/>');
jQuery('.jquery-spelling-container').html(jQuery('textarea').val()).keyup(function(){
    jQuery('textarea').val(jQuery(this).text()); //take the plain text, not the span
    var words = jQuery(this).text().split(' ');
    for (var i = 0; i < words.length; i++)
    {
        if(dictionary.indexOf(words[i]) == -1) // your function here to test for correct spelling
        {
            words[i] = '<span class="jquery-spelling-incorrect">' + words[i] + '</span>';
        }
    }
    jQuery(this).html(words.join(' '));
});