<svg width="600" height="80" xmlns="http://www.w3.org/2000/svg" version="1.1">
  <line class="line" x1="0" x2="600" y1="0" y2="0" stroke="#4af" stroke-width="20" />
</svg>
body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  align-content: center;
  justify-content: center;
}

.line {
  stroke-dasharray: 600;
  stroke-dashoffset: 600;
  animation: offset 2s linear infinite;
}

@keyframes offset {
  to {
    stroke-dashoffset: 0;
  }
}

External CSS

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External JavaScript

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