如何让一个相对定位的孩子获得所有父母的身高

body, html {
  height: 90%;
}
#content{
  width: 100%;
  height: 100%;
}

#sidebar {
  position: relative;
  width: 200px;
  float: left;
  height: 100%;
  background-color: green;
}
#sidebar-content {
  height: 120px;
  background-color: blue;
  
}
#sidebar-footer {
  position: absolute;
  bottom: 0;
  height: 50px;
  width: 100%;
  background-color: black;
}

#main {
  position: relative;
  overflow: hidden;
  background-color: red;
}
#main-content {
   height: 750px;
}body,