Many elements and practices of a web design influence how you publish content on your site, which, however, affects how search engine spiders crawl and index your site. This is something you cannot risk.
If your site on-page SEO fundamentals are not smooth, you'll end up fighting an uphill battle for visibility from the start.
Moreover, aside from content on your site, certain web design essentials directly affect SEO. However, to ensure worthy web design practices (and subsequent search engine visibility), it is important to hire a company with expertise in designing and SEO ranking.