Which HTML5 attribute is used for adding alternative text to an image for accessibility purposes?

The HTML5 attribute used for adding alternative text to an image for accessibility purposes is the "alt" attribute. Here's how you can use it:

1. First, you need to have an image that you want to add alternative text to. This image is added to your HTML document using the <img> tag.

2. The "alt" attribute is added within the <img> tag. For example, if you have an image of a cat, your HTML code might look like this: <img src="cat.jpg">.

3. To add the "alt" attribute, you would modify the <img> tag to include alt="your alternative text". Using the cat image as an example, your HTML code might look like this: <img src="cat.jpg" alt="A cute orange cat sitting on a chair">.

4. The alternative text should describe the image in a way that would make sense if the image wasn't able to be displayed. In this case, if the image of the cat couldn't be displayed for some reason, the user would see "A cute orange cat sitting on a chair" instead.

5. This "alt" attribute is especially important for accessibility purposes, as it allows screen readers to describe images to visually impaired users.