How can I detect DOM ready and add a class without jQuery?

If you want to reproduce the jQuery’s document.ready event, you can use the onreadystatechange or DOMContentLoaded events where applicable:

function domReady () {
  document.body.className += " javascript";
  // ...
}

// Mozilla, Opera, Webkit 
if ( document.addEventListener ) {
  document.addEventListener( "DOMContentLoaded", function(){
    document.removeEventListener( "DOMContentLoaded", arguments.callee, false);
    domReady();
  }, false );

// If IE event model is used
} else if ( document.attachEvent ) {
  // ensure firing before onload
  document.attachEvent("onreadystatechange", function(){
    if ( document.readyState === "complete" ) {
      document.detachEvent( "onreadystatechange", arguments.callee );
      domReady();
    }
  });
}