<style>
body {
    width: 200px;
    margin-left: 100px;
}

.row {
    height: 200px;
    margin-left: -50px;
    background-color: blueviolet;
}
.item {
    float: left;
    width: 50%;
}
</style><div class="row"><div class="item" style="background-color: lightblue">
    a</div>
<div class="item" style="background-color: lightgoldenrodyellow">
    b</div>
</div>