Accessibility is not a style preference. It is the set of things a page must do so that people who are not using a mouse and eyes can still get the same result. The list is short, and most of it is HTML you already know how to write.
About 2 min of reading
Measure what the page announces
Find the controls
Before you run it, say out loud what it will print. Then open the answer.
Find the fakes
Before you run it, say out loud what it will print. Then open the answer.
Find images with no name
Before you run it, say out loud what it will print. Then open the answer.
Tab through it
Before you run it, say out loud what it will print. Then open the answer.
A name for each control, a keyboard path to each control, a reading order that matches the visual order, and a visible focus indicator. Those four cover most failures an audit finds.
It is not a substitute for clear writing, and it is not a legal checkbox. It is a contract about the interface: the same result must be reachable through more than one input channel.
If a task cannot be done with Tab and Enter alone, it cannot be done by a screen reader, a switch device, or someone using a keyboard because a mouse is hard for them. Keyboard is the cheapest proxy for the whole class.
<button> is focusable, pressable with Enter, announces itself as a button, and is disabled by disabled. A <div> with a click handler has none of that. The element choice is most of the fix.
Assistive technology reads a computed name and role for each interactive thing. That name comes from text content, alt, or aria-label. No name means the announcement is a file path or nothing at all.