* {

  padding: 0;

  margin: 0;

}

html {

  box-sizing: border-box;

}

*, *:before, *:after {

  box-sizing: inherit;

}

​

html,body {

  height:100%;

}

​

header {

  background: blue;

  height:50px;

  position:relative;

  z-index:1;

}

​

#main {

  background: red;

  height: 100%;

  position: relative;

  margin-top: -50px;

  padding-top:50px;

}