body height not filling 100% page height

When in doubt - the best way I found to tackle this is by adding:

html { overflow-y:scroll; }

Looks like you have float:left applied on your children. Use this code :

html, body {
    overflow: auto;
}

Please use this answer wisely, I't could help in many cases:

html, body{min-height:100%;}
body{height:100vh;}

try adding

body {
  min-height: 100%;
}