Perceivable

Products used to be built assuming everyone looks at the screen. Quarterly revenue went up as one image, success and failure were a green dot and a red dot, and the walkthrough video had no captions. For someone looking, all of that arrives.

For someone who isn't, the same screen is blank. A screen reader, which reads screen contents aloud, says "image" and moves on, so the numbers inside it are information that doesn't exist. To someone who can't distinguish those colors, both dots are the same grey circle. When the container for a piece of information hangs on one sense, nothing survives for anyone not using that sense. Nobody reports this as a bug either, because the people who can see the screen never encounter it.

Guidelines sit under this principle, and under those sit statements that can be judged true or false. Four items come up most in practice.

1. Give images a text description. Alt text describes an image in words. For a chart, that means what it shows and what the trend is, not the word "chart."
2. Provide information carried in audio as text too. Captions on video, a transcript alongside spoken guidance.
3. Keep enough brightness difference between text and background. Color contrast ratio expresses that difference as a number, and body-size text is generally held to at least 4.5 to 1. Light grey on white sits around 2.8 to 1 and fails, while the same grey two steps darker passes.
4. Don't convey anything by color alone. Pair color with a shape or a word — this overlaps with what color blindness considerations cover. Green with a check for success, red with a cross for failure.

Every item is written as a number or a presence check, so it gets judged against a standard rather than by a person. Removing taste from the argument is the practical value of the principle, and it is why these items get fixed instead of debated.

"This chart fails perceivable. Let's ship the table of figures with it."

Information locked inside an image should exist in another form too.

"Success and failure can't be told apart by dot color alone."

A piece of information hangs on one color, so some people receive none of it.

"This grey is 2.8 to 1, so it can't be body text."

The number is below the threshold and doesn't pass, whatever the palette says.

Pagedeck's document status indicator was caught by this item. Sync complete and sync failed were distinguished only by a green and a red dot, so Noa added a check mark and an exclamation mark beside them. Theo made the same information reach assistive technology as well.