Option 1
Simply add overflow:auto;
to your div
CSS
Demo Fiddle
td
{
vertical-align: top;
}
td.a div
{
background-color: #f00;
height: 100%;overflow:auto;
}
Option 2
Alternatively you'll need to define the height of your table in order for the child to be able to calculate what its 100% is 100% of.
Option 3
The only other way would be to set position:relative
on the td
elements then position:absolute
for the child div