Thanks - well, what I didn't say is that I'm using Calibre to convert my XHTML file, and we get 3 choices for MOBI: 'old,' 'new,' and 'both.' I use 'old' for my MOBI conversion because Calibre warns us that there will be display issues with the other two options in some devices. I mean, sure, s/he could just use bold, or make it bigger, but.using a line or some words in Courier monospaced wouldn't ruin the whole book for someone and in pre-KF8 mobis, the user didn't have the luxury of choosing his or her font anyway they were 'stuck' with Caecilia (TNR clone). Using a different font is an excellent way to signal that, in an environment in which many of the other routes traditionally available to book designers, like using blockquoted-style text, are no longer available to him or her. Or a newspaper article, and he wants to change the headline font, so as to alert the reader that what s/he is reading now is removed from the regular narrative flow. He might have a perfectly good reason to want a slightly different font say the hero stumbles across a road-sign, or a diary entry.

Readers often prefer different fonts than you might. You should really let the user choose the font, if their ereader supports that.

Calibre font specifications