[Parts of this were stolen from @Borror0's answer and Are References Enough?]
Q: What makes a good reference?
A: It is expected that all significant claims should be referenced.
The reference need not be a web-site - it can be printed articles, books, documentaries, etc.
A high-quality reference has, at least, the following characteristics:
- Comes from a peer-reviewed source.
- Is either a primary source, or it, in turn, cites its sources, so the primary sources can be tracked down.
- Supports the argument being provided in the answer (i.e. not out of context)
- Draws logical and statistically robust conclusions from any premises or data it offers.
- the reference is widely available for others to inspect (e.g. not behind a paywall, or out-of-print)
- the reference is not contradicted by similar articles supporting the opposite claim.
The reference need not come from the list of useful sources, but these are widely-respected sources.
Not all references are going to be able to meet this quality bar, but your arguments will be more convincing, and are more likely to receive votes from fellow skeptics when they do.