Semantic Markup

Semantic is the use of a markup language such as HTML to convey information
about the meaning of each element in a document through proper selection of
markup elements, and to maintain complete separation between the markup and the
visual presentation of the elements contained in the document.

Section 508-Accesibility

Section 508 of the Rehabilitation Act requires federal agencies to develop,
procure, maintain, and use ICT that is accesible to people with disabilities.
This law also requires federal agencies to make their ICT available to everyone
, meaning that those who work for the federal government or agencies are
allowed access to equipment, computers, and phones that are available to them.
However, they are limited to these things for designated work.

For a HTML, it is required that you meet standards such as “every image,
applet and embeded media, plug-in, etc. that conveys content has equivalent
alternative text (alt, longdesc, or in the element context)”

SEO-Search Engine Optimization

It is the process of getting traffic from the all major search engines such
as Bing, Google and Yahoo. All of these search engines have primary web
results, where web pages and other content such as videos or local listings are
shown and ranked on what the search engine considers most relevant to users.

How they are related

Combination of Semantic Markup, Section 508-Accesibility and Search Engine Optimization makes your website credible and accesible to a large group of audiance because it fulfills various leagal practices and general practices of an excellent website.