Apps in HTML5

Jeremy Apthorp talked on "The Future of HTML5 Apps" at Linux Conference 2013 today. He described the history of the quest for web based applications which work outside the browser window and work on different operating systems. He described standards work by W3C. He showed how some of the problems of making such apps secure and run offline with Goggle Chrome Packaged Apps. One design decision I did not understand is that remote image files are not permitted, on the basis that the device may be off-line and so not have the image file. However the IMAGE tag of HTML included ALT to provide alternate text for when the image cannot be seen. This could be used when the image in not available. Application designers are required to implement this anyway for accessibility reasons. One curious aspect of this is that Google has two different operating systems (Google Android and Google Chrome) for the Apps to work in.