Don't feel like you have to donate, though. The app is cheap to host (one of the benefits of an offline-enabled, no-accounts-required, optimized-to-the-max web app is that the backend doesn't do much :P), and I'm lucky that MaxCDN & others are providing free service to the app.
What keeps me going is seeing the impact that DevDocs is having (people using and liking it). So the best way to "give back" is to spread the word, send a thank you note, and contribute (one thing in particular that would be great to see is more/better extensions & integrations with code editors).
I've been thinking about writing blog posts on DevDocs's internals and the techniques it uses for a while, but it's hard to find the time, especially when I already spend a lot of my spare time maintaining/updating the app.
That said, feel free to open a GitHub issue to remind me to write blog posts at some point (if there is interest), or if you have specific questions.
Could you specify more on the front-end code on this project, did you hand-roll your own javascript front-end or did you use a framework? The whole thing is rather impressive and I'm looking to do something similar to your sidebar with my own project. Is all of your open source code open to be re-used/modified?
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DevDocs is just a hobby of mine and I don't want to waste my time on a documentation that I may have to remove. If Oracle is willing to grant DevDocs a license (I haven't asked them) I'd gladly add the JDK docs to it.