What relevant CSS rule can't be applied to this? We know our previous context and everything before which is what CSS acts on anyway, no lookahead rules in CSS :)
And yes, there could come an yet as to unknown element later that changes the display, displays on top of using position:absolute; for example, of current content. But not that changes how current content itself is displayed.
"Assume that the amount of HTML currently loaded is finite but sufficient for all of the state to be properly rendered."
So consider <html> <div class="myContainer"> <div>1</div> <div>2</div> <div>3</div> . . .
I've never met an HTML streaming solution that won't just stream this sequentially, which means no </div> for myContainer will ever be emitted (and therefore the HTML will never be well-formed, and the CSS will never have sufficient information to lay out).
If the HTML were streamed such that <html><div class="myContainer"></div></html> were received, and then the interior of myContainer were streamed, that'd be a different story, but that doesn't exist. So I don't think this blog post's argument that CSS3 is Turing complete works for any real implementation.