I am not a developer or a programmer: I can just about create and maintain a plain vanilla website like this one based on very meagre knowledge of HTML and CSS. Many years ago I had bought a couple of books and read them, otherwise I have learnt from looking at other sites, copy and learn as it were, as well as trial and error with lots of errors. If I find anything interesting on a web page, my automatic reaction is to look at the page source and find out more. Presumably many others do the same, and that is why there is a message in the source of pages on the Guardian website. I believe other companies do this as well, as I have heard others mention similar messages appearing on other sites, but this is the first time I have seen it.