How accessibility works
Accessibility is the practice of building a site that people can use whatever their sight, hearing, movement or attention allows. In practice that means the content is available in more than one form and the page can be operated in more than one way: text that a screen reader can announce in a sensible order, controls that work from a keyboard alone, colours with enough contrast to read in daylight, captions on video, and forms whose labels stay attached to their fields.
Most of it comes down to using the right markup for the job. A heading marked as a heading, a button marked as a button, a list marked as a list — this is what assistive software reads to build its own picture of the page. The widely used reference for all this is WCAG, the Web Content Accessibility Guidelines, which many public bodies and larger clients require suppliers to work towards.
Why accessibility matters
The first reason is that a share of every audience cannot use a careless site, and they are not a niche. Vision, hearing and dexterity vary across any population, and temporary situations count too: a cracked screen, bright sun, a slow connection, one hand holding a child.
The second reason is that search engines read pages in much the same way assistive software does. Descriptive headings, meaningful link text, alternative text on images and clean semantic markup help both audiences, which is why accessibility and technical SEO keep arriving at the same fixes. Adding alt text or replacing a link that says “read more” is an accessibility improvement and a search improvement at once.
Common mistakes with accessibility
The most expensive is treating it as a bolt-on. An overlay widget dropped into the footer does not repair markup that was wrong underneath; it sits on top of the problem and gives the owner false confidence. The work belongs in the build, where it is cheap, rather than in a retrofit after launch.
The everyday mistakes are smaller and more common. Headings chosen for how large they look rather than what they mean. Pale grey text on white because it looked refined in the design file. Images carrying important information with no text alternative, including the poster graphics that are so often used for events and offers. Menus and pop-ups that cannot be closed without a mouse. Videos with no captions, which also lose the many viewers who watch with the sound off.
How to act on it
Start by testing rather than guessing. Put the mouse aside and move through a key page with the keyboard alone: if you cannot reach the menu, the form and the send button, that is your first task. Then run an automated checker to catch contrast and missing labels, remembering it will find only the mechanical faults and never judge whether your alt text is useful.
Fix in order of harm: anything that blocks a task outright, then anything that makes content unreadable, then polish. Write image descriptions that say what the image conveys rather than listing what is in it. Use semantic HTML instead of styled divs. Bake the standard into the design stage, because retrofitting an accessible experience onto a finished website build always costs more than doing it once.