CSS best practices
Use a language that eases CSS development and compiles into CSS like LESS or SASS. It's really worth trying! (And there's good tooling support.)
When something is possible to style in a straightforward way without the usage of images by only using CSS (even e.g. by using font icons), then do it from CSS.
Try to avoid HTML markup that serves just to enable some kind of styling.
For HTML classes and IDs use dashed names e.g. this-is-a-class. In Orchard modules you may prefix these with the module name.
This page is part of the Orchard Dojo Library!
This page and many others here are part of the open-source, portable package of Orchard Core goodies: the Orchard Dojo Library for Orchard Core. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!
Note that the Library also has an Orchard 1.x version, check it out here!