<script>
function Show_HidePassword(id) {
element = document.getElementById(id);
if (element.type == 'password') {
var inp = document.createElement("input");
inp.id = id;
inp.type = "text";
inp.value = element.value;
element.parentNode.replaceChild(inp, element);
}
else {
var inp = document.createElement("input");
inp.id = id;
inp.type = "password";
inp.value = element.value;
element.parentNode.replaceChild(inp, element);
}
}
</script>
<input type="password" id="pid">
<a href="#" onclick="Show_HidePassword('pid')"><font color="blue">Показать введенные символы</font></a>

function checkFormChangePassword(form) {
var password = form.password.value;
var confpassword = form.confpassword.value;

var bad = "";
if (password.length == 0) bad += "Вы не ввели пароль" + "\n";
else {
if (password != confpassword) bad += "Пароли не совпадают" + "\n";
if (password.length < min_password) bad += "Минимальная длина пароля: " + min_password + " символов" + "\n";
if (password.length > max_password) bad += "Максимальная длина пароля: " + max_password + " символа" + "\n";
}
if (bad != "") {
bad = "Следующие данные некорректны:" + "\n" + bad;
alert(bad);
return false;
}
return true;
}

<form id = "changepasswordprofile" action = "/update_profile.php" method = "post" onsubmit = "return checkFormChangePasswordProfile(this)">
<table style = "margin: 0 auto;">
<tr>
<td align = "left">
<label id = "laboldpassword">Ваш текущий пароль:</label>
</td>
<td>
<input type = "password" name = "oldpassword" id = "pass1" onfocus = "lightLabel('laboldpassword', true)" onblur = "lightLabel('laboldpassword', false)" />
</td>
</tr>
<tr>
<td align = "left">
<label id = "labnewpassword">Ваш новый пароль:</label>
</td>
<td>
<input type = "password" name = "password" id = "pass2" onfocus = "lightLabel('labnewpassword', true)" onblur = "lightLabel('labnewpassword', false)" />
</td>
</tr>
<tr>
<td align = "left">
<label id = "labconfpassword">Подтвердите новый пароль:</label>
</td>
<td>
<input type = "password" name = "confpassword" id = "pass3" onfocus = "lightLabel('labconfpassword', true)" onblur = "lightLabel('labconfpassword', false)" />
</td>
</tr>
<tr>
<td colspan = "2" align = "center">
<input class = "button" type = "submit" name = "submit_password" value = "Изменить пароль" />
</td>
</tr>
</table>
</form>