A web designer is one of those people who are absolutely necessary for the development of a good website. However, when it comes to SEO, a web designer can be quite difficult to handle. While it is true, it is now quite common to find web designers who are very efficient with SEO, it is equally common to find designers who are so focused with the graphics and visual site that they forget everything else, including usability and SEO.
It is important to make something clear: no matter how good your website looks, if is not optimized, it might not appear in Google searches or Bing.
If you do the SEO of your site on your own, then perhaps you should not worry (in theory) so much for your designer, but there are things that a designer can do that will greatly affect what you do in SEO.
Here I present some of the most common sins committed by some designers: