Display a div width 100% with margins

You can use calc() css function (browser support).

#page {
  background: red;
  float: left;
  width: 100%;
  height: 300px;
}

#margin {
  background: green;
  float: left;
  width: -moz-calc(100% - 10px);
  width: -webkit-calc(100% - 10px);
  width: -o-calc(100% - 10px);
  width: calc(100% - 10px);
  height: 300px;
  margin: 10px;
}​

Alternatively, try using padding instead of margin and box-sizing: border-box (browser support):

#page {
    background: red;
    width: 100%;
    height: 300px;
    padding: 10px;
}

#margin {
    box-sizing: border-box;
    background: green;
    width: 100%;
    height: 300px;
}