Non-blocking javascript and css in modern browsers. Is it still needed?

I am playing a little with some non-blocking JavaScript loading. This means I have a small snippet of JavaScript in my head , and load all my external files at runtime. I even took it a little further to load CSS non-blocking.

Undefined Constant MYSQLI_ASYNC - Non blocking queries in PHP

I understand that mysqlnd allows non blocking queries. However, I'm trying: