What requirements do online publications have to fulfil before they can be deposited?

The German National Library only accepts online publications that have actually been published. Both the content and the format must be identical to the original publication stored on the depositor's server.
You can use web forms to transmit PDF and EPUB files as well as container and/or archive files. You can also use the automated procedures to deposit digital audio books in mp3 or wav format. The preferred file formats for e-books, e-journals, e-papers, online university publications and digital sheet music are published PDF/A and other PDF versions.
In order to guarantee long-term preservation online publications may not be password-protected or copy-protected on submission

More information is available at "File Format Policy for the Delivery of Net Publications":
Dateiformat-Policy für die Ablieferung von Netzpublikationen.