Not exactly with HTML validation but a little JavaScript can resolve the issue, follow the example below:
function check() {
var input = document.getElementById('password_confirm');
if (input.value != document.getElementById('password').value) {
input.setCustomValidity('Password Must be Matching.');
} else {
// input is valid -- reset the error message
input.setCustomValidity('');
}
}
<p>
<label for="password">Password:</label>
<input name="password" required="required" type="password" id="password" oninput="check()"/>
</p>
<p>
<label for="password_confirm">Confirm Password:</label>
<input name="password_confirm" required="required" type="password" id="password_confirm" oninput="check()"/>
</p>
<input type="submit" />