So when it comes time to do accessibility testing, what do we look for? How can I, as a tester, determine whether something is an accessibility issue or not? While it would be best to ask the people with disabilities directly, budget and time constraints can prevent firsthand feedback. A second approach is to emulate some of these disabilities and use other automated testing platforms to determine where the site needs ADA compliance.
Technology is changing, and many website designers are using creative and innovative ways to present web-based materials. These changes may involve new and different access problems and solutions for people with disabilities. This Chapter discusses just a few of the most common ways in which websites can pose barriers to access for people with disabilities. By using the resources listed at the end of this Chapter, you can learn to identify and address other barriers.
In 1998, Congress amended the Rehabilitation Act to require federal agencies to make their electronics and information technology accessible to people with disabilities. It was enacted to eliminate barriers in information technology, to make available new opportunities for people with disabilities, and encourage development of technologies that will help achieve these goals. This pretty much states that agencies and companies must give their employees and members of the public with disabilities access to information that is comparable to the access available to others.

Emotional support animals, therapy animals, and companion animals are seldom allowed to accompany students in public schools. Indeed, the ADA does not contemplate the use of animals other than those meeting the definition of “service animal.”  Ultimately, the determination whether a student may utilize an animal other than a service animal should be made on a case-by-case basis by the IEP or Section 504 team.

Students with disabilities in public schools (K-12) are covered by Individuals with Disabilities Education Act (IDEA), Title II of the ADA, and Section 504 of the Rehabilitation Act. Students with disabilities in public postsecondary education are covered by Title II and Section 504.  Title III of the ADA applies to private schools (K-12 and post-secondary) that are not operated by religious entities. Private schools that receive federal funding are also covered by Section 504.
UPDATE: Since writing this post in August 2017, several important changes have taken place in the laws regarding ADA compliance for websites. On December 26, 2017, the Department of Justice announced that they have withdrawn the Obama-era Advance Notice of Proposed Rulemaking mentioned in this article which intended to require ADA website compliance. The DOJ’s withdrawal announcement stated, “The Department will continue to assess whether specific technical standards are necessary and appropriate to assist covered entities with complying with the ADA.”

Federal law isn't the only consideration for businesses. Additionally, each state interprets the law differently. Consider the case against Netflix in 2012. Lawsuits were brought in federal court in Massachusetts and California. Netflix was accused of violating the ADA by not offering "closed captioning" options for its Internet streamed movies. Illustrating the complexity of this issue, the courts reached completely opposite decisions. Massachusetts held that Netflix must comply with the ADA, while the California court found that Netflix did not fall under the ADA's definition of "public accommodation."
^ Esshaki, Tiffany (July 21, 2015). "Remembering the 'Capitol Crawl'". C&G News. Retrieved January 15, 2016. The event, known as the "Capitol Crawl", was an image that legislators couldn't ignore, Bauer said. She had fought since the 1960s to legally protect the rights of people with disabilities, and with that heroic display, she said, lawmakers simply couldn't go back to their constituents without action.