I agree that Dreamweaver will not invalidate the code. Valid code is good or cross-browser compatibility, not
SEO. The major player in
SEO is IBLs. This has absolutely nothing to do with the site you created, as the IBLs reside on other sites.
A site should already be search engine-friendly when it is designed. Probably the most important items that a designer can contribute with regards to
SEO would be descriptive and brief page titles. As keywords appear to be largely ignored in the meta tags, I wouldn't spend a lot of time there. The description meta tag however has some importance. It is used for indexing as well as a possible source for a synopsis presented in the SERPs. Alt and/or title tags should be used for the main images using related, descriptive terms. Same for links.
The IBLs anchor (linking) text should feature the keyword(s) that you would expect searchers to use to find the site. Again, IBLs are not optimizing the site, they are simply pointing keywords.
Oh, be sure to use actual text that can be indexed and not simply images of text whenever possible.