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.
More resources on this topic:
Now honestly for the platform this person's site was on, they were actually performing rather well. They did however have some real issues going on in the code. Think about all the extra code needed if ever paragraph had a <div> tag that defined each item mentioned above and some having even more.
Now with proper CSS (Cascading Style Sheets) you only have to define all these items once and then apply this to all the content. This is kind of simplified to make it non-techy, but by doing this you could eliminate a ton of extra code in order to keep things clean and easy to use and read for future programmers.
More importantly I would bet this kind of extra code bogs down load speed for browsers and search engines. This isn't even bringing up user issues with slow loading sites.
Here are a few more links that might help:
Granted we all could use some help optimizing our sites and really focusing on load speed in order to make things smoother for visitors and also help our search engine rankings. The point is this, review some of the tools and articles mentioned here and you will be able to get your site ranking better and performing better on search engines in no time.
Good luck and happy coding (or uncoding in some cases)!