Unfortunately I don’t have that book (I thought I did ) so I have to ask a few questions.
Could you explain exactly what you did? Is there a html file with an external css file linked to it? Or is the CSS in the head section of the page itself?
What did you save as html? You don’t want to save CSS on its own as a .html file. You would save it as a .css file. So, that’s why I’m asking for clarification of what you are actually doing there.