Rotating svg elements about their center

See this (resolved as invalid) bug report in Firefox about your problem: https://bugzilla.mozilla.org/show_bug.cgi?id=1209061

Normally, CSS transforms on SVG elements are relative to the viewport and not to the element itself. This can be changed by adding transform-box: fill-box:

svg .rotate {
  animation: rotate 5s linear infinite;
  transform-box: fill-box;
  transform-origin: center;
}

@keyframes rotate {
  from { transform: rotate(0deg); }
  to { transform: rotate(360deg); }
}

Working fiddle: https://jsfiddle.net/g9zfcdm3/10/

Background

From the MDN docs about transform-box:

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

border-box (default)

The border box is used as the reference box. The reference box of a 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.

Note that it's an experimental feature and probably won't work in IE and Edge.