Focus states
A visible focus state is not decoration. It tells keyboard users where they are. Do not remove it without replacing it.
Labels
Every form input needs a label. Placeholder text is not a label. aria-label is a fallback, not a default.
Semantic markup
Use button for buttons, a for links, and headings in order. It costs nothing and helps screen readers.
Summary
Accessibility is not a separate project. It is a set of small choices made while building.