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Science-backed data on why AI says what it says
Browser usability

Image alt text

Alt text is the text alternative supplied through an image's alt attribute. It should convey the image's purpose or information in context. A deliberately empty alt attribute can be appropriate for a decorative image.

Checked against BLURSOR’s measurements ·

What it means

The right alternative depends on why an image is present. A product photograph may communicate a feature or colour. A chart may need a nearby explanation of its data. A background flourish may add no information at all.

Missing alt and empty alt are different. Omitting the attribute leaves the intended treatment unclear; an empty value deliberately supplies no alternative text. That can prevent decorative images from adding noise, but it is unsuitable when the image carries information the visitor needs.

Illustrative example

Hypothetical example: a notebook product page

Imagine a product image that shows a notebook's elastic fastening and inside pocket. A useful alternative might identify those features if the surrounding text does not. A decorative ink splash behind the product title could instead have an empty alternative.

A size diagram needs more than a filename such as 'diagram-final.png'. Put the relevant dimensions in readable text as well. These are invented editorial examples, not measurements or prescribed wording for a real product.

How BLURSOR measures it

In the report: Images without text alternatives

BLURSOR's missing-alt fraction counts inspected images whose alt attribute is absent. In the live probe, hidden images and images explicitly marked as presentational are excluded. Images with alt set to an empty string are not counted as missing.

The detector does not judge whether an alternative is accurate, useful, or appropriate for the image. It also does not recognise everything that conveys visual information: its image count is based on img elements, rather than a complete review of CSS backgrounds, canvas drawings, diagrams, or every possible image state. HTML estimates have different visibility limits.

What to do next

  1. Ask the content owner to classify each important image as informative, functional, or decorative before writing its alternative.
  2. Describe the relevant information or action, and avoid filling alternatives with filenames or repetitive promotional keywords.
  3. For complex charts and diagrams, provide the substantive explanation or data in nearby readable content and check it with someone who cannot see the image.

What it cannot tell you

  • Zero missing attributes can coexist with poor alternatives or informative images incorrectly given empty values.
  • The result is a bounded markup check, not a complete accessibility assessment or evidence that an AI system understood the pictures.

Sources and further reading

The definitions below describe the underlying concepts. The measurement section describes BLURSOR’s own checks.

Related explanations