Responsive shine animation using linear-gradient as background

Question

I want to create a shine loading animation which will appear on multiple elements with different background colors.

Currently, I'm using background-image gradient and I'm animating the background-position using vw units, but it's not scalable, my elements will have different lengths.

Is there a way I can animate background-image with percentage units?

The animation created

body {
  background: black;
}

header {
  width: 100%;
  height: 50px;
  background-color: rebeccapurple;
  background-image: linear-gradient(
    to right,
    transparent 0%,
    rgba(255,255,255,0.3) 50%,
    transparent 100%
  );
  background-repeat: no-repeat;
  background-position: -100vw;
  animation: shine 2s infinite;
}

@keyframes shine {
  0% {
    background-position: -100vw;    
  }
  100% {
    background-position: 100vw;   
  }
}

<header></header>

Answer 1

An idea is to make the size of the gradient to be 3 times bigger than the container and color the middle part of it then you slide it from left to right:

body {
  background: black;
}

.box {
  height: 50px;
  margin:5px;
  background: 
    linear-gradient(90deg,#0000 33%,rgba(255,255,255,0.3) 50%,#0000 66%)
    rebeccapurple;
  background-size:300% 100%;
  animation: shine 2s infinite;
}

@keyframes shine {
  0% {
    background-position: right;    
  }
  /*100% {
    background-position: left; it's the default value, no need to define it
  }*/
}

<div class="box"></div>

<div class="box" style="width:60%"></div>

<div class="box" style="width:40%"></div>

Another alternative for a different animation:

body {
  background: black;
}

.box {
  height: 50px;
  margin:5px;
  background: 
    repeating-linear-gradient(90deg,#0000 0,rgba(255,255,255,0.3) 25%,#0000 50%)
    rebeccapurple;
  background-size:200% 100%;
  animation: shine 1s infinite linear;
}

@keyframes shine {
  0% {
    background-position: right;    
  }
}

<div class="box"></div>

<div class="box" style="width:60%"></div>

<div class="box" style="width:40%"></div>

Related question: Using percentage values with background-position on a linear-gradient