Just How Long Should Alt Text Be?

I teach a class over at the local college here in Long Beach and a majority of the content is hosted on the Canvas LMS so students can access it online. And, naturally, I want the content to be as accessible as possible, so thank goodness Canvas has a11y tooling built right into it.

But it ain’t all that rosy. It makes assumptions like all other a11y tooling and adheres to guidelines that were programmed into it. It’s not like the WCAG is baked right in and updated when it updates.

The reason this is even on my mind is that Jeremy yesterday described his love for writing image descriptions:

I enjoy writing alt text. I recently described how I updated my posting interface here on my own site to put a textarea for alt text front and centre for my notes with photos. Since then I’ve been enjoying the creative challenge of writing useful—but also evocative—alt text.

I buy into that! Writing alt text is a challenge that requires a delicate dance between the technical and the creative. It’s both an opportunity to make content more accessible and enhance the user experience.

One of those programmed guidelines in the Canvas tool is a cap of 120 characters on alt text. Why 120? I dunno, I couldn’t find any supporting guideline or rule for that exact number. One answer is that screen readers stop announcing text after 125 characters, but that’s apparently untrue, at least today. The general advice for how long alt text should be comes in varying degrees:

  • Jake Archibald talks of length in terms of emotion. Detail is great, but too much detail might distort the focal point, which makes total sense.
  • Dave sees them as short, succinct paragraphs.
  • Carrie Fisher suggests a 150-character limit not because screen readers will truncate them but more as a mental note that maybe things are getting too descriptive.
  • Daniel Göransson says in this 2017 guide that it comes down to context and knowing when certain details of an image are worth additional explanation. But he generally errs on the side of conciseness.

So, how long should alt text be? The general consensus here is that there is no hard limit, but more of a contextual awareness of what purpose the image serves and adapting to it accordingly.

Which gets me back to Jeremy’s article. He was writing alt text for a group of speaker headshots and realized the text was all starting to sound the same. He paused, thought about the experience, compared it to the experience of a sighted user, and created parity between them:

The more speakers were added to the line-up, the more I felt like I was repeating myself with the alt text. […] The experience of a sighted person looking at a page full of speakers is that after a while the images kind of blend together. So if the alt text also starts to sound a bit repetitive after a while, maybe that’s not such a bad thing. A screen reader user would be getting an equivalent experience.

I dig that. So if you’re looking for a hard and fast rule on character counts, sorry to disappoint. Like so many other things, context is king and that’s the sort of thing that can’t be codified, or even automated for that matter.

And while we’re on the topic, just noticed that Twitter has UI to display alt text:

Now if only there was more contrast between that text and the background… a11y is hard.


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