An accessible site is one a person can use with a keyboard, a screen reader, or poor sight. I build and fix to WCAG 2.2 Level AA. That is still the W3C recommendation, and it is the standard UK public-sector sites have to meet.
A private business is not under that same regulation. The Equality Act 2010 does require reasonable adjustments for disabled customers. Meeting 2.2 Level AA is the practical way to show that.
Structure
Use the HTML element that matches the job. A button is a button, a heading is a heading, and the headings stay in order. A screen reader uses that structure to move around the page. A div with a click handler does not give it that.
Images and colour
Informative images need alt text that says what the image is for. Decorative images get an empty alt attribute, so they are skipped.
Body text needs a contrast ratio of at least 4.5 to 1 against its background. Large text and interface parts such as input borders need 3 to 1. Colour cannot be the only way to show an error or a state. That rule has been in WCAG since 2.0. It was not introduced in 2.2.
Keyboard and focus
Every action has to work from the keyboard, in an order that follows the page. The focus indicator has to stay visible. WCAG 2.2 adds that something else on the page, such as a sticky header, must not cover the focused item.
If an action depends on dragging, there has to be another way to do it. Clickable targets should be at least 24 by 24 CSS pixels. That minimum is a 2.2 rule.
Forms
Every field needs a visible label, tied to the input. Placeholder text disappears as someone types, so it is not a label. Errors need to be written out, not only turned red.
WCAG 2.2 also asks that help, where you offer it, sits in the same place from page to page, and that logging in does not depend on a memory test such as a puzzle or a transcribed code. A password manager has to be able to fill the form.
How I check
I go through the page with the keyboard, check contrast, and run axe or Lighthouse. Those tools miss things, so they are a start. On a new build I also pass the main journey through a screen reader: VoiceOver on a Mac, NVDA on Windows.
If a form or a page is already hard to use, tell me what breaks and I will look at that path first.