A formal proof is a sequence of statements where each is either an axiom or derived from previous ones via a set of derivation rules, and wherein assessing the validity of each axiom statement or derivation can be done "typographically" (mechanically and primitive-recursively). It's nigh-impossible to write a human-readable formal proof of anything interesting; the vast majority of mathematical papers published are informal proofs.
(Of course, this diversion doesn't detract from the matter of this proof-poem being very cool.)
[trying to get the rhyme going for you]
The idea would be that we generally share understanding of obvious axioms (and thus, allow them to be request-able assumptions), and so can enhance readability.
Do you know about Metamath yet? It's something similar to what you want, but it's all in a single program as opposed to being web-based.