Things are said to happen "on" a website. Even within a website items are placed "on" pages. This is consistent with the original concept of a website as being an online publication written in Hyper Text Markup Language or HTML.
Conventional markup is the process used by editors to correct and format printed publications before they are typeset and items in books or other publications are always said to appear "on" a given page.
Information is sometimes said to appear "in" a book, magazine or newspaper but this is because conventional documents have covers which have to be opened before you can see what's "on" the pages. However websites have no covers: as soon as you connect to the site you are presented with a page with information "on" it, thus the normal preposition used in relation to websites is "on".