In my case, it's "whatever I am currently trying to learn". And I re-engineer a lot (I try for every couple of years)
Re-designing websites with new tech stacks is, from a business or time-efficiency standpoint, a terrible idea, but from a learning standpoint, it's great. You get to work on something you are passionate about, which keeps you invested in the learning.
Right now I'm upgrading to Rails 5 so that I can convert most of the views to Preact.
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