No content on a webpage should start or change unexpectedly.
When someone has difficulties with reading or concentration, anything moving on the screen can cause distractions. These distractions can prevent them from completing tasks or simply getting the information needed from a website.
For people who are susceptible to seizures, avoid any page elements which have strobing, flickering, or flashing effects.
For people with vestibular issues this content can cause dizziness and nausea.
Assistive technology can have problems presenting content that changes unexpectedly.
Use the WCAG links provided to get detailed guidelines on how to achieve accessibility for this scenario.