I polished the technique a bit more to solve previous issues:
To better detect browser capabilities, I used a transparent gradient as a secondary background image. That makes old Webkit browsers (which solves problem with Android 2.x) and IE9 (capable of multi-background but not animations) to use the Gif Fallback.
background: transparent url(images/loading.gif) center center no-repeat;
background-image: linear-gradient(transparent, transparent), url(images/loading-static.svg);
In addition, I used CSS animations since the SVG animation was problematic on Safari.
In theory (based on browser version
support for gradients and
svg), it should work across all browsers (some SVG-capable old browsers may still use the fallback but it guarantees that SVG is not used by browsers not able to render it).
Pau