When we use attribute alt=" "

Question

about <img>

Answer 1

'alt' attribute provide description of image... valid html require it, even you set it with an empty string (but it's not advised and should be reserved for layout decorative related ones: the rule you can follow is to ask you if blind people would need to have image described for 'read'/understand your content (voice synthetizer could so describe correctly the context of page)...

Answer 2

using alt is what will show up if the image is not found -- usually gives a description of what the image is

Answer 3

If the image can't be found or loaded, it'll display whatever text you put inside of it. Alternate to the image.