Semantic HTML is the use of HTML markup to reinforce the semantics, or meaning, of the information in webpages. Search engines use semantic markup to figure out what a page 'means' so they show up when a user searches up a keyword or term. This mainly focuses on the look or presentation of the web page which is processed by the web browsers internet user choose.
Section 508 is an amendment to the United States Workforce Rehabilitation Act of 1973, which is a federal law mandating that all electronic and information technology developed and used by the federal government be accessible to people with disabilities.
SEO or Search Engine Optimization is the name given to the activity that attempts to improve search engine rankings. It is the simple activity of ensuring a website can be found in search engines for words and phases relevant to what the website is offering.
They are all related in terms that they work together in order to give users a successful working webpage. When a website developer starts on a website, they should keep the semantic markup in mind. With keeping the semantic markup and Section 508 (Rehabilitation Act of 1973) in mind, they will be able to create a nice looking website that will give them a higher chance of being on the first page of search engine results due to Search Engine Optimization.