-gtk-icon-transform doesn't work in css?

[MartyMcFlyInTheSky]

From the documentation I get this:

While eww supports a significant portion of the CSS you know from the web, not everything is supported, as eww relies on GTK's own CSS engine. Notably, some animation features are unsupported, as well as most layout-related CSS properties such as flexbox, float, absolute position or width/height.

So I thought I could make use of some GTK3 features to implement an icon spinner like this:

$spinner-size: 50px;
$spinner-color: #007bff;
$spinner-border-width: 4px;
$spinner-animation-duration: 1s;

// Spinner element
.spinner {
  border: $spinner-border-width solid rgba($spinner-color, 0.2);
  border-top: $spinner-border-width solid $spinner-color;
  border-radius: 50%;
  animation: spin $spinner-animation-duration linear infinite;
}

// Keyframes for the spin animation
@keyframes spin {
  0% {
    -gtk-icon-transform: rotate(0deg);
  }
  100% {
    -gtk-icon-transform: rotate(360deg);
  }
  // to { -gtk-icon-transform: rotate(1turn); }
}

The GTK3 css documentation states this should work. Also this stackoverflow post seems to indicate that the property works. However, for me it only displays a static icon, nothing is animated. Are those css properties not passed throught to GTK or what could be the culprit? The commented out code as a replacement also doesn't work.