If their primary task was to build HTML emails then I could understand that train of thought.

I suppose there are people out there that are working as front-end developers for websites that only work in HTML and CSS, but I would think that's getting more rare as time goes on. Of course, that could just be bias based on my own career.

But, going with your suggestion as a thought exercise, I would go in a different direction. To make a pattern like fizzbuzz I suppose I would do something more like: build a page with a series of paragraphs so that these CSS properties apply to every third paragraph, these CSS properties apply to every fifth paragraph, and so on. Something possible with CSS but requires the ability to think things through.