There are many Search Engine Optimization design factors which involve how your HTML is structured (e.g. H1 headings, meta tags, alt tags, etc.) But beyond this alphabet soup of technical factors, a lot of SEO comes down to good copywriting. If you have compelling content which by design contains key words and phrases which you're targeting, you have a foundation for good SEO ranking. Some reason that copywriting is the single most important aspect of SEO. Here's why:

Companies interested in increasing sales rely on copywriting to monetize those leads.

Poor copywriting may attract a high volume of qualified buyers but ones who aren't being convinced to buy your product or service.

Poor copywriting is usually done by people so focused on rankings that they forget about the conversion value of the text (read by real potential customers!) and fixate on tweaking the web page copy solely to please the search robots.

Quality copywriting has headlines and text that snags your attention and entices you to read more. The copy quickly helps the reader to understand that the product meets their needs and answers the questions they had in mind when they typed in their keywords into the search engine.
By priming the customer with good copy, the calls to action like "Buy Now" or "Contact Us For a Free Consultation" the customer will gladly do so because the quality copy has persuaded them in a way poor text could not.

Read more about copywriting and Search Engine Optimization Information.