I have a question concerning the admirable Stacks Project.
Which comparable projects are there:
approach-wise: "an open source textbook on algebraic stacks and the algebraic geometry that is needed to define them", i.e. projects as "an open source textbook on X and the Y that is needed to define (and understand) X"
technical-wise [Side question: which technical framework(s) is the Stacks Project based on?]
tag-system-wise (using the same tag system as the Stacks Project)
tag-wise (with a significant number of tags shared with the Stacks Project)?
The question concerns also the possible interoperability of several such projects.