Accessibility is usually presented as an ethical duty owed to a minority of users. That picture is both incomplete and the reason the work keeps sliding down the priority list.
In practice most accessibility work makes a site better for everyone. Someone squinting at their phone in direct sunlight has the same problem as a user with low vision — just temporarily.
Contrast
WCAG asks for a contrast ratio of 4.5:1 for normal text and 3:1 for large text. Light grey on white — fashionable for years — almost always fails.
This can't be judged by eye. On this site, every time a background changed I re-measured, and more than once a figure came back under the threshold when nothing looked wrong at all.
A visible focus ring
Plenty of templates remove the browser's focus ring because it's “ugly”. The result is that anyone navigating by keyboard has no idea where they are. If you don't like how it looks, restyle it — don't delete it.
The keyboard
A simple test: put the mouse aside and move through the whole site with Tab and Enter. If you get stuck, or can't close a menu, you've found a real defect.
Alt text
Not every image needs it. A decorative image should carry an empty alt so screen readers skip it. An image carrying information should say that information in words — not repeat the filename.
The commercial part
Search engines read the same structure a screen reader does: real headings, meaningful alternative text, and links whose destination is clear from their wording. A site a screen reader can follow is a site Google can follow.