http://jsbin.com/slideUpUsingMinHeight/
.maincontentdiv {
position:relative;
height:200px;
width:200px;
background:red;
}
.slideup {
position:absolute;
width:100%; /* Absolute el. loose width so... */
bottom:0;
min-height:0; /* Use min- height instead */
background:blue;
transition: min-height 250ms ease-in; /* target min-height respectively */
}
.maincontentdiv:hover > .slideup { /* hover el > children selector */
min-height: 50%; /* and animate! */
}