.ClassNameA {
    right: 0;
    bottom: 0;
    left: 0;
}
.ClassNameB > * {
    width: 100%;
    height: auto;
    position: absolute;
    top: 0;
}

.ClassNameA {
    right: 0;
    bottom: 0;
    left: 0;
}
.ClassNameB > * {
    width: 100%;
    height: auto;
    position: absolute;
    top: 0;
}