Because WordPress is popular with top designers, you would expect custom designers to know how to achieve optimum WordPress SEO. And indeed they do know what works for best WordPress SEO. Actually, the same things that make great SEO on any site work to enhance WordPress SEO. Good clean code makes great SEO. W3C validation helps to achieve that. External style sheets help keep the code clean, and are the way to go for WordPress SEO.
Great content with good WordPress GPL themes makes great SEO too. Whether your WordPress site is a blog, a web-commerce site, or a community service project, you want to be found. The best of all ways to bring traffic to your site is to offer great content. Content that is complete, on topic, and valuable to your readers means repeat visitors as well as lots of linking. And that is good WordPress SEO.
There are a number of more technical tricks for achieving maximum WordPress SEO success. Your talented designer will know them, and will put them to good use. Titles can be optimized, links need to checked as well, and image optimization is very important to WordPress SEO. There are ways to limit duplicate content in archives which helps greatly with WordPress SEO.
Why is WordPress the choice of so many top designers and high-value customers?
Because it combines terrific usefulness as a content management system with a very customizable appearance. Because it is strong and safe, as well as stable and reliable, that is another reason why WordPress is so often chosen for CMS installations. WordPress looks as good as it is, and really is as good as it looks. That is why WordPress is the best choice for your CMS project.