G'day artperl,

SVG is a W3C Recommendation which is documented in Scalable Vector Graphics (SVG) 1.1 (Second Edition).

Chapter 6, Styling, describes both the styling properties shared by CSS as well as those unique to SVG.

The property which seems most relevant to your question is fill-opacity. This is not a CSS property so it won't appear in the CSS documentation (if that's where you looking).