split3

<style type="text/css">
/* Panels: */
.panel{ 
  padding: 1em; 
  border-width: 6px; 
  border-style: solid; 
  height: 4em; 
}

/* Resizing */
.resize-x { 
  resize: horizontal;
  overflow: auto;
}

/* Panel Colors */
.box-a{ border-color: #F6F2A4; background: #FFFEDB;}
.box-b{ border-color: #F2B98D; background: #FEE5D0;}
.box-c{ border-color: #F18FB3; background: #FDD1E2;}

/* Initial Dimensions */
.p-25{ width: 25%; }
.p-50{ width: 50%; }
.p-75{ width: 75%; }

/* Split View */
.split-view {
  margin: 1em 0; 
  width: 100%;
  clear: both;
  display: table;
}

.split-view .panel {
  display: table-cell;
}
</style>

<div class="split-view">
  <div class="resize-x p-25 box-a panel">Panel A</div>
  <div class="box-b panel">Panel B</div>
</div>