Not all issues are equal, and the ranking that matters is not severity in the abstract but what a person cannot do because of it. A missing alt attribute on a decorative image costs nothing; an unlabelled checkout field costs a sale and sits under the same criterion. This page ranks the common issues by consequence, defines the term everybody meets first, and says what people with visual impairments actually need, which is broader than screen readers.
The issues ranked by what they cost
First, anything that makes a task impossible: an unreachable control, a keyboard trap, an unlabelled required field, a modal that never returns focus. Second, anything that makes a task confusing: errors in colour only, unnamed buttons, missing headings. Third, everything that degrades the experience: poor alt text, low contrast on secondary text, link text that says here.
What alt text means, and when it should be empty
Alt text is a text alternative announced in place of an image. It should say what the image contributes, not describe every pixel: a photo of a product needs the product named, a chart needs its finding stated in the page. Decorative images take an empty alt attribute so they are skipped entirely, and getting that distinction right removes most of the noise a screen reader user hears.
What visual impairment website accessibility actually needs
Far more people magnify, increase contrast or reflow text than use a screen reader. That means text that resizes to 200% without loss, a layout that reflows at 320 pixels wide, contrast that meets the ratio, and no meaning carried by colour alone. Those four help the largest group and are the easiest part of all of this to test.
Questions people ask about accessibility issues
What does alt text mean in practice?
One sentence carrying what the image contributes. If the page already says it, the image is decorative and takes empty alt.
Should we write alt text for every image?
Write it for every meaningful image and empty alt for decoration. Both are decisions; neither is a blank attribute.
Which issue should we fix first?
Whichever makes something impossible on a path a customer uses.