Effective Alt Text

"Alt text" (alternative text) is a critical aid for making online content accessible to vision impaired readers; it provides a text description for images that will be picked up by screen readers. The most important thing is that you get in the habit of creating alt text, but it's also good to consider how best to write it. It's easy to fall into the trap of overwriting in a way that slows things down or is redundant to other text (such as a caption) or even editorializes beyond the scope of what the image would reasonably convey on its own. Don't feel the need to use it for design elements; think about whether or not the picture adds to the message or context of your piece.

See this article [PDF] for some examples, and good tips like the following:

Describe the content of the image without editorialising. Say what you see – and don't make assumptions about ethnicity, gender, what's happening out of shot, a subject's motivations etc.

But do be descriptive about what you can see – adding information about skin tone, hair colour/style etc. will help to build the picture for someone who can't see the image.

One of the best suggestions I’ve heard is to think how you’d briefly describe the image over the phone.

Never start with “Image of …” or “Picture of …”

It’s going to be obvious to either a person or a machine when something they're accessing is alt text.

Imagine how frustrating it'd be using a screen reader on an image-heavy page, and having it read: "Image of the theatre" "Image of front of house area" "Image of exterior signage" "Image of the box office team" "Image of the auditorium" "Image of the stage" … arrrrgh!

However it is good to help people understand context, so explaining the type of image – e.g headshot, illustration, chart, screengrab – can be useful.

See this article [PDF] for more specific guidance on dealing with information-dense images, like graphs. They recommend very concise alt text, but with a detailed description in the body of the main text. This is a good rule to follow even outside of vision concerns. When you make a graph, it's obvious to you what it represents, but anyone encountering it for the first time may find it helpful for you to walk them through how to interpret it.