Einfaches Kontaktformular mit Validierung

HTML

<form>
    <div>
        <label>Name (Pflichtfeld):</label>
        <input name="name" type="text" required>    
    </div>
    <div>
        <label>E-Mail (Pflichtfeld):</label>
        <input name="mail" type="email" required>    
    </div>
    <div>
        <label>Nachricht</label>
        <textarea name="nachricht" required></textarea>    
    </div>
    <div>
        <button>Absenden</button>
    </div>    
</form>

CSS

body {
    font-family: Verdana, sans-serif;
    font-size: 13px;
}

label {
    display: block;
    margin-bottom: 5px;
}
div {
    margin-bottom: 10px;
 }

:required {
    border: 2px solid #a20000;
    background-color: rgba(162,0,0,0.25)
}


/* Das geht leider nicht! */
:required + label {
    font-weight: bold;
    color: #a20000;
}