There are two online, evolving, collaborative "foundational text" projects for research mathematicians that I am aware of:
(1) The Stacks Project for algebraic geometry
(2) Kerodon for categorical homotopy theory
I find them admirable, valuable and suited to our times -They are sort of Bourbaki for the internet age. I'd like to know if there are others of the same nature that are ongoing or in the offing.
Please note that I am not looking for texts or monographs available online which is why I have highlighted the adjectives in the first line.