Definition:
Alt is an attribute HTML specified in the IMG tag to provide alternative text when no image of a web page can not be displayed due to some kind of technical problem (such as an old browser, tracking protection or a faulty link) or to describe such image to people with reduced visibility. It is a fundamental element for the positioning, usability and accessibility of our website and therefore we must take care that all the images on our website have it.
The ALT attribute in SEO
In SEO positioning techniques, an alt attribute related to the keyword can help to better position the page in search engines. Search engines as a general rule can not read images so thanks to this type of files they can identify the content of the images and index them in the most appropriate way. If we want to implement the ALT attribute properly, it is recommended that it contains the keyword of the website while making an accurate description of the image.
The ALT attribute as a visual aid
The alt attribute does not always have to literally describe the content of the image. You have to take into account the purpose and context of the image and what would be useful for someone who can’t see it. The alt attribute is supposed to be an alternative to the image, usually its purpose must be declared. For example, an image of a warning sign should not have as an alt attribute the text “a triangle with a yellow background, black border and an exclamation mark”, but simply “Attention!” (Unless, of course, the purpose of the alt text is to actually show what the warning symbol looks like.