I'd recommend that you never set a style on the base div, span, or img tags.
To a lesser extent, you may also need to take care about what style properties you use with the body tag and lists (ol/ul/li).
If you're producing HTML5, topics in the output will contain a whole load of additional HTML on the page that isn't in your source - it'll add HTML for the skin, proxies, etc.
So what you put in your stylesheet is applied to the whole page, including all the other stuff that's only in the output - so it's easy to apply styles to things you didn't intend to.
My experience is that there's lots of information on the style part of CSS and not nearly enough on the cascading part.
I can hack my way through adjusting styles when I need to, but I'm always tripping over just where the tags are, or should be, applied.