Indeed, the reason is simply to keep the current look-ahead(1) for the CSS syntax. If you allow nested rules with selector list which doesn't start with a symbo

Indeed, the reason is simply to keep the current look-ahead(1) for the CSS syntax. If you allow nested rules with selector list which doesn't start with a symbol, you need (in theory) an unbounded look-ahead. However, the current spec has been written in a way that it doesn't prevent removing this restriction in the future. But the CSSWG would prefer to have some real world profiling data to know the actual performance cost of removing this restriction.