Position absolute but relative to parent

I have two divs inside another div, and I want to position one child div to the top right of the parent div, and the other child div to the bottom of the parent div using css. Ie, I want to use absolute positioning with the two child divs, but position them relative to the parent div rather than the page. How can I do this?

Sample html:

<div id="father">
   <div id="son1"></div>
   <div id="son2"></div>
</div>

#father {
   position: relative;
}

#son1 {
   position: absolute;
   top: 0;
}

#son2 {
   position: absolute;
   bottom: 0;
}

This works because position: absolute means something like "use top, right, bottom, left to position yourself in relation to the nearest ancestor who has position: absolute or position: relative."

So we make #father have position: relative, and the children have position: absolute, then use top and bottom to position the children.

div#father {
    position: relative;
}
div#son1 {
    position: absolute;
    /* put your coords here */
}
div#son2 {
    position: absolute;
    /* put your coords here */
}

Incase someone wants to postion a child div directly under a parent

#father {
   position: relative;
}

#son1 {
   position: absolute;
   top: 100%;
}

Working demo Codepen

If you don't give any position to parent then by default it takes static. If you want to understand that difference refer to this example

Example 1::

http://jsfiddle.net/Cr9KB/1/

   #mainall
{
   
    background-color:red;
    height:150px;
    overflow:scroll
}

Here parent class has no position so element is placed according to body.

Example 2::

http://jsfiddle.net/Cr9KB/2/

#mainall
{
    position:relative;
    background-color:red;
    height:150px;
    overflow:scroll
}

In this example parent has relative position hence element are positioned absolute inside relative parent.

In a use case of needing a sticky floating "back button"/ "back to top" button. But you have the main content area that can shrink in favor of the side content area

You can use position: fixed; inside a container of position: absolute; to get a more flexible behavior similar to sticky but more powerful

function toggleOpen() {
  const element = document.getElementById("sideContnet");
  if (element.style.display === "none") {
    element.style.display = "block";
  } else {
    element.style.display = "none";
  }
}

.container {
  display: flex;
  margin: 0 auto;
    width: 600px;
}

.contentBig {
  width: 600px;
  min-width: 66%;
  position: relative;
  
}
.contentSmall {
  width: 230px;
  min-width: 33%;
}

.absolute {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  width: 60px;
}

.fiexBack {
  position: fixed;
  background-color: red;
  bottom: 20px;
  padding: 5px;
}

.button {
  width: 200px;
  padding: 12px;
  background-color: coral;
  margin: 0 auto;
}

<button onclick="toggleOpen()" id="toggleSide" class="button">
  toggle open side content
</button>
<div class="container">
  <div class="contentBig">
    <div class="absolute">
      <div class="fiexBack">Back</div>
  </div>
    <p>
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     </p>
<p>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus vehicula est at pretium venenatis. Morbi mauris justo, viverra non velit sit amet, accumsan cursus nisi. Pellentesque ut vulputate sem. Etiam sit amet quam diam. Nulla vel sodales est. Quisque sed accumsan urna. Ut tristique efficitur ante a congue. Cras elementum dignissim tellus, in rhoncus ex faucibus nec. Sed vitae mi eu leo interdum aliquam. Donec eleifend nisl sem, a tincidunt velit commodo sit amet. Nulla ex eros, tempor in fringilla ut, accumsan ac augue. Curabitur aliquet venenatis massa, ornare viverra lorem varius interdum.
     </p>
<p>
Morbi leo nulla, varius nec dignissim quis, vestibulum quis mi. Sed dignissim lobortis magna. Ut erat nisl, varius id finibus at, faucibus scelerisque justo. Donec viverra purus eu ante volutpat iaculis. Donec vehicula ullamcorper urna ut egestas. Curabitur convallis at risus vitae fermentum. Nullam arcu ante, faucibus quis neque vel, pulvinar blandit est.
     </p>
<p>
Curabitur auctor ipsum ac interdum accumsan. Sed quis arcu mauris. Maecenas nibh ligula, tristique rhoncus pharetra vel, blandit non lectus. Suspendisse orci felis, faucibus sit amet rhoncus eu, ullamcorper et nulla. Ut in leo eu risus dignissim tempus sed sit amet leo. Etiam pulvinar lectus tincidunt turpis viverra maximus. Donec rutrum rutrum dui sit amet congue.
     </p>
<p>
Morbi leo nulla, varius nec dignissim quis, vestibulum quis mi. Sed dignissim lobortis magna. Ut erat nisl, varius id finibus at, faucibus scelerisque justo. Donec viverra purus eu ante volutpat iaculis. Donec vehicula ullamcorper urna ut egestas. Curabitur convallis at risus vitae fermentum. Nullam arcu ante, faucibus quis neque vel, pulvinar blandit est.
     </p>
  </div>
   <div id="sideContnet" class="contentSmall">
    <p>
       Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus vehicula est at pretium venenatis. Morbi mauris justo, viverra non velit sit amet, accumsan cursus nisi. Pellentesque ut vulputate sem. Etiam sit amet quam diam. Nulla vel sodales est. Quisque sed accumsan urna. Ut tristique efficitur ante a congue. Cras elementum dignissim tellus, in rhoncus ex faucibus nec. Sed vitae mi eu leo interdum aliquam. Donec eleifend nisl sem, a tincidunt velit commodo sit amet. Nulla ex eros, tempor in fringilla ut, accumsan ac augue. Curabitur aliquet venenatis massa, ornare viverra lorem varius interdum.
     </p>
<p>
Morbi leo nulla, varius nec dignissim quis, vestibulum quis mi. Sed dignissim lobortis magna. Ut erat nisl, varius id finibus at, faucibus scelerisque justo. Donec viverra purus eu ante volutpat iaculis. Donec vehicula ullamcorper urna ut egestas. Curabitur convallis at risus vitae fermentum. Nullam arcu ante, faucibus quis neque vel, pulvinar blandit est.
     </p>
  </div>
</div>

Full Code Example