Re: Theme designers: Use CSS classes, not IDs.
That’s part of it yes…
…but also, using ID based formatting can cause trouble down the line when users want to add more elements to the page and find they have to change the CSS rather than just saying that their new addition
<thing id='latest' />; should be formatted in the same way as
<thing id="popular" />; because they’re both the same kind of table.
This is a double pain because CSS seems to get stuck in some browser caches so users see the new page with the old CSS.
The shift from “how do I want this to look?” to “what kind of thing is this and what do those things look like?” is (I have found) worthwhile.