<animate>

html,
body,
svg {
  height: 100%;
  margin: 0;
  padding: 0;
}

<svg viewBox="0 0 10 10" xmlns="http://www.w3.org/2000/svg">
  <rect width="10" height="10">
    <animate
      attributeName="rx"
      values="0;5;0"
      dur="10s"
      repeatCount="indefinite" />
  </rect>
</svg>