I agree it's a big mistake to not make the nesting rules compatible to Sass/Less.

> Because of limitations in browser parsing engines, you must make sure the nested selector (.bar in the above example) always starts with a symbol.

How hard can it be.. I know that is always easy to say from the outside but in this case I don't understand why the refuse to implement it. Just labelling it as a performance problem doesn't say much because why should it impact performance to check whether there appears another rule with an opening "{" after it.