Semantic markup is using HTML to show how words are used on a web page. You use different tags to organize and markup the page. There are several different types of semantic tags such as, headers, footer, main, aside, nav, mark, section, etc. You can use h1 in brackets to show a header and h2 and h3 for different sections.
508 accessibility means that a technology can be used and accessed by anyone even if they have a disability. They are set by Section 508 of the Rehabilitation Act of 1973. This means that federal agencies are required to provide software and website accessibility to people who have disabilities. Making a website 508 accessible means that they are compatible with assistive technology. You can use templates like VPAT to help create a service that is 508 accessible.
SEO stands for search engine optimization. It is a way to make your website visible and towards the top of a web search engine without having to pay for a top result spot. The higher up your website is on the search engine list, the more visitors it will receive. You can also use it for images, news, videos, etc. You can edit a websites HTML to optimize it's search because you can put in keywords to increase it's relevance.
How They All Relate
Semantic markup helps with 508 accessibility because it organizes the page so their disablility technology, like their page readers, can work. Semantic markup also helps with search engine optimization so that it can help figure out what is on the page. You can add keywords in the HTML during the semantic markup so that it adds to the search engine optimization.