7.4.1. Indicate in the alt attribute of the CAPTCHA image where the user can find the non-graphic CAPTCHA version

Whenever a CAPTCHA image is used, indicate where the non-graphic version of the CAPTCHA is found in the alt attribute of the image.

Often, the non-graphic version is an audio version. In this case you could have an alt text similar to this:

<img src="captcha.png" alt="CAPTCHA (antispam image). If you cannot see the image, use the audio version available below." />