The International Court of Justice was asked for an advisory opinion on the question of Kosovo’s secession declaration. The request came from the General Assembly, as drafted by Serbia, and the framing of the question is partly why I title the post “sort of” rules for Kosovo. The question was framed as whether the secession declaration was “in accordance” with international law.
The short answer is that the ICJ ruled narrowly, on grounds widely accepted by most international lawyers across a wide spectrum of political views. Viz., a secession declaration is not a subject of international law, so it is not contrary to international law, but for the reason that international law does not address the issue. But my co-blogger at Opinio Juris, Chris Borgen (an expert in secession issues and particularly the former Soviet bloc), explains all this in a pair of posts here and here; likewise Marko Milanovic at the European Journal of International Law blog in a detailed, lawyerly post.