DevDocs is an Electron-based application which enables offline documentation browsing for many web and other development technologies.

The app has support for C, C++, CoffeeScript, CSS, DOM, Git, HTML, HTTP headers and status codes, JavaScript, jQuery, Markdown, PHP, Python, React, Ruby, SVG and more (there are around 200 virtual manuals in total).

All the available documentations are listed in a left-hand sidebar. Click "Select documentation" bottom-left, check the boxes next to anything you need, click Save and the new technologies are collected together at the top of the sidebar.

You can browse them much like any other web-based manual. Expand something like HTTP, select an interesting section such as Status and click a code to find out what it means. Or just use the fuzzy search feature to find all potential matches.

Initially you're browsing web content on devdocs.io, but an Offline feature enable downloading some or all of the sections.

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A straightforward tool which brings most of the manuals you need into one place. Whether you really need it is more open to question. It might be more convenient to access the documentation on the DevDocs website, or via the Chrome extension.