<label for="first-name">Your phone number:</label><br>

<input type="tel"
       name="phone"
       minLength="9" maxLength="9"
       placeholder="It must be 9 digits">

input {
    margin-top: 0.5rem;
}

input::placeholder {
    font-weight: bold;
    opacity: 0.5;
    color: red;
}