Well, I'm a little confused, if you are typing both, then auto will preside over 100%. If you want the height 100%, it's only possible with a parent having a fixed height. 100% is not a fixed height. Now if i remember correctly, you need to set body,html {height:100%;} for a div inside to be set 100%. Now that hasn't worked for me always in all browsers, besides that I'm not sure.