We start out with the intention of jumping into HTML & CSSing the Top Treehouse Ad that we just designed, but of course get derailed as soon as we start with fiddling around with the logo and how it behaves in the responsive environment. It's appropriate timing though, as we'll need to really define the space we have available for the top ad as we start building it.
When we do start building the top ad, we make a new file to write all our ad-related CSS, _ads.scss.
By the end, we have the markup in place and the basic positioning ready. We just need to apply the finishing touches through CSS yet.