In the interim, what I did was to transcribe the letter, then mark it with
<div class="write"> to indicate that it should be rendered as handwriting.
There are other "slate exercises" in the book that I have not yet marked up,
and I will probably treat them in the same way.

The associated CSS file specifies the font to use (cursive), which you are
apparently unhappy with.

For what it's worth, my browser renders the "Dear Santa" letter in Italic Comic Sans. :-/