The Best Way to Use Alt Text on Logos in HTML

Google has discussed the best way to handle the addition of alt text for logos and image-based buttons. It is most beneficial to describe what the button does or what the image on the button is, in the situation where the button is used as an icon. The rules may appear to be complicated, but they are actually quite easy to understand. The World Wide Web Consortium (W3C) has published an explainer on how to handle logos in HTML. The correct way to use alt text on images such as logos actually depends on whether the image is a link or not. If the logo image functions as a link back to the home page, then it is correct to label that image with the function that it has, so that a site visitor using a screen reader can know that this logo is a link to the home page. If an image cannot be displayed on a page for some reason, the alt text associated with that image will be displayed instead. It is therefore important to ensure that the alt text accurately reflects the content and function of the image. For example, if an image is a button that will trigger an email when clicked, the alt text should say something like "Email button" or "Click to email." The purpose of the alt text is to ensure that visitors to the page can still understand its content and functionality, even if the images are not visible.

(source: https://www.searchenginejournal.com/alt-text-for-logos-and-buttons/469801/)