Lately, the internet has served about every actual researcher in the understanding of basic concepts on their fields. Then, why can’t we take it as a valid source on scientific papers?
The most important reason I can find, is the dynamic behavior of the net. It evolves as time passes by. This is, there is no tracking of the quality and veracity of information in time. Books, on the other hand, are hard printed, and even if an old book contains unaccurate data at a time, it is data we can check against. On the internet, most of the time, updted data completely replaces old data without leaving a trace to reference miscalculations or falacies.
nanoHUB, provides researchers and people with a source of valuable data that can be relied on, due to version tracking and stability. Even if it is a web source, many people have cited its resources on their projects.
Still, internet sources will hardly ever replace paper.
A web page consumes costly energy to exist. It can disappear any time. A shelf is all a book needs to exist for decades.