Triangle CSS

HTML

<div class="test">
    Ma boîte
</div>

CSS

.test {
    width: 200px; height: 100px;
    background-color: grey;
    position: relative;
}
.test::after {
    display:block;height:0;width:0;content:"";
    border-top: transparent solid 20px;
    border-left: green solid 20px;
    border-bottom: transparent solid 20px;
    position: absolute;
    top: 30px; right: -19px;
}
body {
    padding: 30px;
    background-image: url(http://azrael.sha.univ-poitiers.fr/~ydelmas/exemples/grille.png);
}