<style>
    .uno {
        width:300px;
        height:100px;
        background-color: red;
        position: relative;
        overflow: hidden;
    }
.due {
    width: 100px;
    height: 100px;
    transform: rotate(45deg);
    background-color: white;
    margin-left: -50px;
    position: absolute;
    top:0;
    left:0;
}
</style>
<div class="uno">
    <div class="due"></div>
</div>