That bothered me too. Trying a few though, it looks like they are 404s. Which is an interesting way to preserve a historical, but no longer valid link. If you are going to the trouble, it might be better to rewrite the links to point to Archive.org or something.
Linking an archived copy (and proactively making an archive submission when initially linking, which seems to be an increasingly common practice), would be preferable.
Because they are broken, apparently. The CSS class for those is `broken_link`, and I tried on the "Primary and secondary compounds", sure enough it leads to an "access denied" response 403.
jacobr1|3 years ago
dredmorbius|3 years ago
smeagull|3 years ago
kroltan|3 years ago