JSFiddle - React, Tailwind, and code Playground

HTML

<form action="send_message.php" method="post">
<table class="form_table">
<tr>
<td style="font-weight:bold;">Subject:</td>
<td><input style=" width:300px" name="form_subject"/></td>
<td></td>
</tr>
<tr>
<td style="font-weight:bold;">Message:</td>
<td id="myWordCount">(300 words left)</td>
<td></td>
</tr>
<tr>
<td><input type="hidden" name="sender_id" value="<?php echo $sender_id?>"></td>
<td><textarea cols="50" rows="4" name="form_message"></textarea></td>
<td valign="bottom"><input type="submit" name="submit_message" value="send"></td>
</tr>
</table>
</form>

JavaScript

var content;
$('textarea').on('keyup', function(){
    // count words
    var words = $(this).val().split(/\s+/).length;
    $('#myWordCount').text("("+(301-words)+" words left)");
    // limit message
    if(words>=301){
        $(this).val(content);
        alert('no more than 300 words, please!');
    } else {    
        content = $(this).val();
    }
});