I think starting from the font tag is historically linked with the worries about browser support people had a few years back. Designers were wary of going full CSS but saw the benefit of using it for styling fonts. This isn’t a valid issue anymore, so I think a clean start is a better approach.

Talking about general concepts is far easier for a wider audience to understand – a lot of people I talk to now are print designers wanting to get an intro into web design and font tags would mean little to them. Information on styling text should ideally start from a more general standpoint, and make a side note about the dreaded font tag as a warning to those users who have used it in the past.

Are you thinking of writing a new book on web design with CSS?