Semantic markup reinforces the meaning of the web page, not the appearance of the page. It basically is the structure of the web page, and separates the different sections in an HTML document. Semantic markup is the underlying meaning of the content and how the conent of the web page is sectioned off. This is important as it ensures your web page is formatted properly across all web browsers.
Section-508 accessibilty is the requirement that all federal agencies provide IT services that are accessible by anyone with a disability. It is strucutered so that anyone can use federal web pages.
Search Engine Optimization
Search engine optimization is defined as the process of maximizing the amount of visitors to a website, by ensuring the particular site appears high on the list of results after being searched in a search engine. This business practice is used to increase traffic to their website in order to promote their business.
How They Are Related
Semantic markup and Section-508 are related because the markup is what makes it appealing and easy to use for users, which makes is what Section-508 is all about since it ensures that people with disabilities are able to use the websites. This is related to search engine optimization because the markup of the web page determines how high it will be on the list of search results.