Pieter Belmans introduced tree view pages to the Stacks project. For example the tree view for Chapter 8: Brauer groups (link and click expand) looks like this:

• Tag 073X points to Section 8.1: Introduction
• Tag 073Y points to Section 8.2: Noncommutative algebras
• Tag 073Z points to Definition 8.2.1
• Tag 0740 points to Definition 8.2.2
• Tag 0741 points to Definition 8.2.3
• Tag 0742 points to Definition 8.2.4
• Tag 0743 points to Definition 8.2.5
• Tag 0744 points to Section 8.3: Wedderburn’s theorem
• Tag 0745 points to Lemma 8.3.1
• Tag 0746 points to Lemma 8.3.2
• Tag 0747 points to Theorem 8.3.3
• and so on

This tree view shows in particular that we have tags for sections, and it allows you to quickly find those tags. The tree view of chapter n is at

http://stacks.math.columbia.edu/chapter/n

The depth of the trees is at the moment at most 3. Since we don’t allow nested environments in the stacks project (exception: equations), I think the maximal depth we can ever get is 5.

The tree view hints at browsing the stacks project online. When eventually mathjax (or whatever will be the next math rendering tool) matures enough to have good support for xypic (and better font size handling), we’ll go and actually set this up.