No, it's a choice of the browser's designers. There's no requirement that browsers display alt text while images are loading. Only if the image ultimately fails to load, or if the image is in an unsupported format, is alt text required. Of the browsers installed on my system, Firefox displays alt text while the image is loading, Konqueror doesn't, and Lynx displays alt text but doesn't load the image at all until you tell it to (making the alt text a lie in this case). In this unscientific survey, only one-third of browsers actually do what you're expecting.