How to Correct Unexpected Animation Behavior in SVG Sub-Elements with `transform-box: fill-box;`?

CSS transform-origin issue on SVG Sub-Element

When attempting to animate a sub-element within an SVG, the element's transform origin is often set relative to the entire SVG rather than the specific sub-element being animated. This can result in unexpected animation behavior, such as an element appearing to scale from the top-left corner of the SVG instead of its own center.

To address this issue, CSS provides the transform-box property which can be used to define the box that will be affected by the transformation. By setting transform-box to fill-box, the transformation will be applied only to the content box of the element (i.e., the area occupied by the element's visible content).

Consider the below example, where a rectangle (id="animated-box") within an SVG is scaled using CSS animation:

@keyframes scaleBox {
  from {
    transform: scale(0);
  }
  to {
    transform: scale(1);
  }
}

#animated-box {
  animation: scaleBox 2s infinite;
}

Without setting the transform-box property, the rectangle will scale from the top-left corner of the SVG, as its transform origin is set to (0,0) relative to the entire SVG.

To correct this behavior and have the rectangle scale from its own center, add transform-box: fill-box; to the #animated-box selector:

@keyframes scaleBox {
  from {
    transform: scale(0);
  }
  to {
    transform: scale(1);
  }
}

#animated-box {
  transform-box: fill-box;
  animation: scaleBox 2s infinite;
}

Now, the rectangle will scale from its own center, as the transform-box property ensures that the transformation is applied only to the element's content box. This provides greater control over the animation and allows for more precise and visually appealing results.

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