A site can appear tidy and be difficult to use for those who surf with the keyboard or with assistive technologies. Accessibility is about the ability to perceive, understand, and use content and functions. It is a quality of digital service.
The W3C provides an introduction to web accessibility and the needs of people with disabilities. From this reference, I propose to consider relevant alternative texts, title structure, contrast and operability of commands. These are aspects to be evaluated in context, not an automatic declaration of conformity.
A hypothetical example is a button indicated only by an icon. Those who do not recognize its meaning may not know what action it takes. An understandable label helps, along with proper management of status and interactions.
Verification should follow real paths: find an article, fill out a form and download a document. Automatic tools can spot some problems, but they do not cover the entire assessment. Adequate human control is also needed. Improving accessibility means removing concrete obstacles, keeping the verifications carried out distinct from any regulatory obligations that require a specific examination of the site and service.