*{margin:0;padding:0;}
.a{width:300px;height:300px; background:#ccc;position: relative;}
.b{width:100px;min-height:50px;max-height:100px; background:red;position: absolute;margin: auto;top: 0;left: 0;right: 0;bottom: 0;}
.c{width:300px;height:300px; background:#999;line-height:300px;text-align: center;}
.d{width:100px;min-height:50px;max-height:100px; background:red;display:inline-block;}