Although authors cannot count on a specific font being used in a document, they can very easily specify generic font families to be used. This particular behavior is very well supported, since any user agent that didn't let authors (or even readers) assign fonts would quickly find itself out of favor.
As for the other areas of font manipulation, support varies. Changing the size of fonts usually works well, but 20th-century implementations ranged from frustratingly simplistic to very nearly correct in this area. The frustrating part for authors is usually not the way in which font sizing is supported, but instead in how a unit they want to use (points) can yield very different results in different media, or even different operating systems and user agents. The dangers of using points are many, and using length units for web design is generally not a good idea. Percentages, em units, and ex units are usually best for changing font sizes, since these scale very well in all common display environments.
The other frustration is likely to be the continued lack of a mechanism to specify fonts for downloading and use in a document. This means that authors are still dependent on the fonts a user has available, and that they cannot predict what appearance that text will take.
Speaking of styling text, there are ways to do so that don't involve fonts, which the next chapter will address.