CSS is not turing-complete - it is as turing-complete as a simple TXT file. Deciding whether some sequence of bytes resembles a CSS file is very possible. Deciding whether a stylesheet can be successfully applied to a website is also decidable. Of course, it is undecidable whether a certain pattern emerges if a stylesheet is applied to an infinitely large HTML file. But then, it is also undecidable whether some TXT file appears in an infinitely large HTML file.