Curve, well there are lots of ways to check various things, but the key in the long run is to cleanup the cart code as much as possible.
To check the HTML code of your site for validation, you can go here.
Result for http://www.curve-appeal.com/ - W3C Markup Validator, that's just your home page.
Second, in pulling your code, there appear to be a number of images without ALT tags, or information in the ALT tags.
Regarding the CSS or use of, they do use it, but the actual CSS is in the files, not pulled from an external CSS file, which is also a concern as loading this much code on each page will slow your site down to some degree, perhaps not visually noticeable, but it does affect things. Reducing the CSS in each page will also improve the content to code ratio of the page, improving your search results.
These are items that as your business grows, you'll want to do what you can to improve them, and tune them, and get your site rising up the SERPS. The other key is that you'll want to begin tune and tweak as you move forward. This is a core planning piece to long term success as a business. It is neither inexpensive or an overnight fix.
While much of this could be completed fairly smoothly, there are lots of things that are long term, ongoing efforts to your success. With some work, things can come through nicely.