A professional agency will apply expert coding and HTML validation, sadly, most developers today just make sure that the "on page"; or what you see, looks good to you, with little or no regard to the codes validation or proper execution and most importantly, the SEO flags. We do not use WYSIWYG website develoment programs. We do not use it's sister environment either, WordPress; which is a plug-in, plug and play website builder which is not much better than a WYSIWYG program. These environments add hundreds of lines of superfluous, useless code to a website, which will cause a web site to load slowly, this is a Google rankning flag, and marks off for a slow loading site. Additionally, Wordpress is not Search Engine friendly. There are WP plug-ins for SEO, but gernerally, they do not work well if at all.
Wikipedia Web Design - full article - Web design encompasses many skills and disciplines in the production and maintenance of websites. The different areas of web design include web graphic design; interface design; authoring, including standardized code and proprietary software; user experience design; and search engine optimization. Often many people will work in teams covering different aspects of the design process, although some designers will cover them all. The term web design is normally used to describe the design processes on the front-end (client side/what you see) design of a website including writing mark up. Web design partially overlaps web engineering in the broader scope of web development. Web designers are expected to have an awareness of usability and if their role involves creating mark up then they are also expected to be up to date with web accessibility guidelines.