Oh!Thank you for this I was really hoping that placeholders could be used as it did seem that this format could then be used to create multimedia webooks that would be compatible on all web devices. This is something that looks as if it is becoming available via sites such as Pressbooks.com which seem to use Wordpress based plug-ins to create web books.
I understand that I can use a standard web site approach but to my naive eyes I thought this might offer an advantage over pdf but it doesn't look like it does now.
Thanks again for clarifying,
Pwalder