Semantic Elements in HTML are elements that successfully describe their meaning to both the browser and the developer simultaneously. Semantic markup is the usage of markup language, like HTML, to convey information about the meaning of each element in a document. In other words, it uses HTML elements to show how the words are used on a page. This helps the search engines figure out what the page means when it shows up on a browser and semantic markup is currently supported in all modern browsers. It important to use because if a page utilizes poor markup, it will be invisible within the search results.
Section 508 is an amendment to the United States Workforce Rehabilitation Act, which mandates that all electronic and information technology that is developed, procured, maintained or used must be accessible to people with disabilities. Accessibility refers to a website being able to support and inhibit social inclusion for people with disabilities as well as others, such as older people, people in rural areas, and people in developing countries. The scope of this amendment is limited to the federal sector and it includes binding, enforceable standards, as well as compliance reporting requirements and a complaint procedure.
SEO (Search Engine Optimization)
SEO is short for search engine optimization and represents the process of getting traffic from the search results organically from search engines. In search results, search engines tend to display pages that are considered to be relevant and authoritative. Essentially it ranks the terms in webpages.
How are they related?
Semantic markup, Section 508, and SEO are related in that they all are utilized to create more relevant, superior, and more used web content. Websites with correct well done semantic will show up more frequently and higher within the search engine. Correct semantic markup will also make a page Section 508 accessible and compliant with US regulations.