While HTML has gone a long way, if you want reasonably stable rendering of pages, PDF is still king of the hill. But being widespread does not ensure stability.
If you look at the these two renderings, in particular around the feet, you will see some pretty weird differences (click to see full-size pictures). The first is a rendering in Safari, while the second was rendered in Chrome. You can notice many gray artefacts in transparent areas of the second image, the colours are also overly saturated.
These differences seem to be only the tip of the iceberg: at this year’s CCC there was an interesting presentation about hybrid file formats, including PDF files that would render different things depending on the viewer.