How to Write a 21st Century Proof

Objective

The objective is to build an HTML element that implements the behaviour described in How to Write a 21st Century Proof, namely:

[…] With hypertext, there is no problem adding extra detail like this. We could add enough levels to reduce the reasoning to applications of elementary axioms. The reader can stop opening lower levels of the proof when satisfied that she understands why the statement is true.

Result