by

The by attribute specifies a relative offset value for an attribute that will be modified during an animation.

The starting value for the attribute is either indicated by specifying it as value for the attribute given in the attributeName or the from attribute.

You can use this attribute with the following SVG elements:

• <animate>
• <animateMotion>
• <animateTransform>

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
  <rect x="10" y="10" width="100" height="100">
    <animate attributeName="width" fill="freeze" by="50" dur="3s" />
  </rect>
</svg>