Although a site uses some CSS– that does not mean that it is optimized or following web standards.
Boy today I was working on a website migration (moving from one platform to another). The first page I created, I just did a copy and paste. Wow what a mess. Every paragraph was styled (defining the fonts to display, defining color of fonts, defining size, etc., etc.). Web standards endorse a set of standardized best practices for building web sites, and a philosophy of web design and development that includes those methods. Usually this require CSS to focus on styling the page and HTML to focus only on content and no design elements.