Well, it is ok as long as you give credit to Nature. I don't.

I am usually highly suspicious about free material on the internet. I founs several well written and illustrated free soruces about European history that were in fact cover-ups for fascist or racist groups.

The only online ressources i use come from universities and i usually confront 3 or more sources before starting to use that kind of material... some specialists can be very opiniated.

Lately i found several closed ressources in free acces like the Littré dictionnary for french language (if you are looking for a word which has not be pronounced since four centuries, you'll find it there) or open ressources in universities FTP and i was really pleased. It took me a bit more time to find, but time is allways a good investment.