what is this alt="" used for
It has 3 usage: 1.(this is not for what it was built, this use became famous because of google and few search engine's facility to provide saved copies of sites) when the site is down and you want to read few articles of it, Google provides the saved copy of that site, with no images mostly so if you have used alt=""then reader will get what kind of picture was there. 2.while loading an image the reader get pre idea about what kind of image it is. 3.in case browser is set on "text-only" version also user will able to get what kind of image was there. And can load selected images only. 4.for blinds if text to speech is on they will get idea that there is image after this text which is related to some xyz kind of topic.
alt="" is basically use when the internet connection of user is too slow and unable to load the image or the image display is blocked by the browser then alt="image_details" i.e the image details written inside parenthesis display.
alternative text when image is not loading
name of photo. use with img