transform-box

The transform-box CSS property defines the layout box to which the transform and transform-origin properties relate.

/* Keyword values */
transform-box: content-box;
transform-box: border-box;
transform-box: fill-box;
transform-box: stroke-box;
transform-box: view-box;

/* Global values */
transform-box: inherit;
transform-box: initial;
transform-box: unset;

The transform-box property is specified as one of the keyword values listed below.

content-box

The content box is used as the reference box. The reference box of a <table> is the border box of its table wrapper box, not its table box.

border-box

The border box is used as the reference box. The reference box of a <table> is the border box of its table wrapper box, not its table box.

fill-box

The object bounding box is used as the reference box.

stroke-box

The stroke bounding box is used as the reference box.

view-box

The nearest SVG viewport is used as the reference box. If a viewBox attribute is specified for the SVG viewport creating element, the reference box is positioned at the origin of the coordinate system established by the viewBox attribute, and the dimension of the reference box is set to the width and height values of the viewBox attribute.

content-box | border-box | fill-box | stroke-box | view-box

In this example we have an SVG:

<svg id="svg" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 50 50">
  <g>
    <circle id="center" fill="red" r="1" transform="translate(25 25)" />
    <circle id="boxcenter" fill="blue" r=".5" transform="translate(15 15)" />
    <rect id="box" x="10" y="10" width="10" height="10" rx="1" ry="1" stroke="black" fill="none" />
  </g>
</svg>

In the CSS we have an animation that uses a transform to rotate the rectangle infinitely. transform-box: fill-box is used to make the transform-origin the center of the bounding box, so the rectangle spins in place. Without it, the transform origin is the center of the SVG canvas, and so you get a very different effect.

svg{
  width:80vh;
  border:1px solid #d9d9d9;
  position:absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

#box{
  transform-origin:50% 50%;
  /*+++++++++++++++++++++++++++*/
  /* if I remove this rule the pen won't work properly on Chrome for Mac, FF, Safari
  Will still work properly on Chrome for PC & Opera*/
  transform-box: fill-box;
  /*Alternatively  I can use transform-origin:15px 15px;*/
  /*+++++++++++++++++++++++++++*/
  animation: rotateBox 3s linear infinite;
}

@keyframes rotateBox {
  to {
    transform: rotate(360deg);
  }

Full credit for this example goes to Pogany; see this codepen for a live version.

• Using CSS transforms
• transform, transform-origin