[md]

.checked:before {
    content: '';
    width: 16px;
    height: 8px;
    display: block;
    position: absolute;
    bottom: 15px;
    left: 6px;
    border-width: 2px;
    border-style: solid;
    border-color: transparent transparent #2d9f05  #2d9f05;
    transform:rotate(-48deg);
    -ms-transform:rotate(-48deg);   /* IE 9 */
    -moz-transform:rotate(-48deg);  /* Firefox */
    -webkit-transform:rotate(-48deg); /* Safari 和 Chrome */
    -o-transform:rotate(-48deg);    /* Opera */
}

效果：