How can I make a line shrink towards the right-hand side?

I have a horizontal line which I am animating. The final part of the animation shrinks the line to a length (width) of zero. However, my code makes it shrink towards the centre, but instead I want the line to shrink towards the right-hand side. How can I achieve this?

body {
  margin: 0;
  background: black;
}

.white-line {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 5px;
  height: 10px;
  background-color: red;
  transform: translate(-50%, -50%);
  animation: expandLine 0.3s ease-in-out forwards 1.2s, moveUp 1.5s ease-in-out forwards 1.41s, contractLine 0.5s ease-in-out forwards 3.3s;
  z-index: 1000;
}    

@keyframes expandLine {
  100% {width: 100%; opacity: 1;}
}

@keyframes moveUp {
  100% {top: 0%;}
}

@keyframes contractLine {
  100% {width: 0;}
}

<div class="white-line"></div>

Solution

The reason it is shrinking towards the centre is because that’s where the line is positioned, at left: 50%. So, while shrinking the width you’ll need to simultaenously move the position of your line across to the right side, or left: 100%.

In your contractLine animation, add the declaration left: 100%.

body {
  margin: 0;
  background: black;
}

.white-line {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 5px;
  height: 10px;
  background-color: red;
  transform: translate(-50%, -50%);
  animation: expandLine 0.3s ease-in-out forwards 1.2s, moveUp 1.5s ease-in-out forwards 1.41s, contractLine 0.5s ease-in-out forwards 3.3s;
  z-index: 1000;
}    

@keyframes expandLine {
  100% {width: 100%; opacity: 1;}
}

@keyframes moveUp {
  100% {top: 0%; transform: translate(-50%, 0);}
}

@keyframes contractLine {
  100% {width: 0; left: 100%;}
}

<div class="white-line"></div>

I have also added transform: translate(-50%, 0) in the moveUp animation, which prevents half the line disappearing off the top of the page.