Superscript in CSS only

Question

How do I implement superscript using only CSS?

I have a stylesheet that uses a superscript character to indicate external links, but I'm having trouble getting the character to align properly.

What I now have is as follows:

a.external:after {
  font-size: 50%;
  vertical-align: top;
  content: "+";
}

However, it is ineffective.

Naturally, I'd only employ the sup>-tag if the material supported HTML...