I was meaning if the designer sets "sans" (or serif or fantasy, etc.) then the user doesn't know the origin of the actual font choice (their browser default and OS installation) but will assume that the specific font used was that chosen by the designer. Most users won't compare a page on multiple OS, those that do may still not realise the source of the displayed font (they could assume the designer chose different fonts for different browsers or OS, say) and so would not know how to alter which font is used.
There are many situations when DejaVu Sans, Helvetica, and Arial look equally good. The print designer notion that the design must look identical everywhere is obsolete for the web, which remains a medium controlled by the user. Jakob Nielsen has written a ton on this subject.
Was that in response to my comment - I'm intensely aware of the pitfalls print designers fall into having remonstrated with several against static design and over specification of visuals.
Nielsen is good (perhaps a tiny touch purist for me), I seldom disagree, however, with the overarching theme of the Alertbox posts.
The web in theory is a user-controlled medium, in practice it is controlled by a mixture of web designers and browser makers; browser defaults of appearance and behaviour are pretty convergent. Like I intimated, the common user barely knows the term "browser" never mind where the font settings are. Whilst one can overload a pages CSS with one's own stylesheet nobody appears to do that.
