HMTL部分

<a href="#" class="close"></a>

CSS部分

.close {position: absolute;right: 32px;top: 32px;width: 32px;height: 32px;
}
.close:before, .close:after {position: absolute;left: 15px;content: ' ';height: 33px;width: 2px;background-color: #333;
}
.close:before {transform: rotate(45deg);
}
.close:after {transform: rotate(-45deg);
}