Ahem helps write predictable tests. The font's em square is exactly square. Its ascent and descent is exactly the size of the em square. The font's extent is exactly the same as its line-height, it can be exactly aligned with padding, borders, margins.
In this article, we will reconcile the designer's requirement for accuracy with the user's need to resize text on demand, arriving at a best practice that satisfies designers and users and works across browsers and platforms.
sIFR (or Scalable Inman Flash Replacement) - method to insert rich typography into web pages without sacrificing accessibility, search engine friendliness, or markup semantics