What is the purpose of the alt attribute on images?

The alt attribute provides alternative information for an image if a user cannot view it. This can be due to connectivity issues, browser limitations, HTTP errors, or if the user is using a screen reader. Not providing an alt attribute will result in a poor user experience for those who cannot view the image.

<img src="image.jpg" alt="Alternative text">

How to write good alt text

The alt attribute should be used to describe the image in a way that makes sense to someone who cannot see it. Descriptions must be accurate and concise. Some screen readers are known to cut off descriptions after 125 characters, so it is best to keep descriptions short.

Let's look at an example. Imagine that you want to describe an image of a boat sailing on the ocean. You could use the following alt attribute:

<img src="boat.jpg" alt="A boat sailing on the ocean">

In this example, we've tried to describe the image as if we were describing to someone over the phone. Notice how we didn't use the word "picture" or "image" in the description. Additionally, we didn't simply say "boat" because that doesn't provide enough context.

What if the image is purely decorative?

If an image is purely decorative, then it is best to leave the alt attribute empty. This will tell screen readers to skip over the image.

<img src="decorative.jpg" alt="">

Notice in this example that we still included the alt attribute, but we left it empty. This is different from not including the alt attribute at all. If you don't include the alt attribute, then screen readers will read the image's file name instead.