<form name="ajout" method="post" action="index.php?uc=marque&action=validajout" onsubmit="javascript:return estValideMarque()" align="center">
    <h3>Ajout d'une marque</h3>
    <input type="text" name="libelle" placeholder="Libellé de la marque"></input>
    <button type="submit" name="valider">Valider</button>
</form>