height 100% doesn't work, two divs inside div

Answer 1

as stated in other answers, the 100% don't work unless the parent height is also specified.

looks like you are trying to achieve 'equal column height', you don't have to set the parent height for that. using CSS table layout can be helpful here.

notice how clean this styling is. no floating elements, and no need to specify the same height again and again in different elements.

look at this Fiddle

best practice: separate your styling from your markup.

HTML:

      

       First to set height
      
      

      Why don't get the 100%
    

CSS:

#Parent
{
    display: table;
}
#Parent > div
{
    display: table-cell;
    border: 1px solid black;
}
#Parent > div:first-child /*if you want to fixed the first column height*/
{
    height: 150px; /*or any other fixed height you want*/
}