The use of sources that are solely available on the Internet carries certain risks. It can be difficult to judge who is disseminating the information on the Internet, what their motives are and what the information is based on. There are no guarantees about the quality or the authenticity of online information. Another problem is the instability of information on many websites. Information can change without notice and entire websites can disappear without a trace.
When using (historical) sources on the Internet (treatises, diplomatic documents, historic speeches, treaties, etc.) it is very important to trace which institution published a particular source and what original they used.
The Internet is a great source for quick fact-finding and getting a handle on your topic, but it does carry risks. When researching politically sensitive topics in particular, the Internet can be useful for finding divergent opinions about that topic, but it is seldom the most suitable place to find factual information.