12345678,
<!doctype html>
<html lang="ru">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#Message{
color: #ff0000;
}
</style>
<script>
document.addEventListener("DOMContentLoaded", function() {
let form = document.forms.form3;
let textarea = form.membershortdescription;
let msg_area = document.getElementById("Message");
let count = document.getElementById("count_display");
let min = 51;
const validate = function(event) {
let {length} = textarea.value;
let correctly = length >= min;
let submit = event.type == "submit";
msg_area.innerHTML = correctly ? "" : submit ? "Слишком мало символов" : `<br>${min - length} осталось`;
count.innerHTML = correctly || submit ? "" : `${min} символ минимум`;
correctly || event.preventDefault();
}
form.addEventListener("submit", validate)
form.addEventListener("input", validate)
});
</script>
</head>
<body>
<form action="mypage.php" name="form3" enctype="multipart/form-data" method="post" >
<textarea rows="8" cols="38" name="membershortdescription" id="membershortdescription" class="profileFormSelect" ></textarea>
<br/>
<span id="count_display">51</span>
<span id="Message">символ минимум</span>
<input type='submit' value='submit'>
</form>
</body>
</html>