kaioa.com

Opera is currently a bit ahead here and there. In this case it's using a span's class as content. Usually Firefox (or Gecko for that matter) can do this as well, but it doesn't want to do it, if they were injected into the DOM via JS.

This is how it's supposed to look like. For getting that kind of effect simply add the following CSS to your theme:

pre.sh_sourceCode span:hover{
  content:attr(class);
}

Don't forget to remove it once you're done. ;)