The coding style used for the Taskwarrior, Taskserver, and other codebases is deliberately kept simple and a little vague. This is because there are many languages involved (C++, C, Python, sh, bash, HTML, troff and more), and specіfying those would be a major effort that detracts from the main focus which is improving the software.
Instead, the general guideline is simply this:
Make all changes and additions such that they blend in perfectly with the surrounding code, so it looks like only one person worked on the source, and that person is rigidly consistent.
To be a little more explicit, the common elements across the languages are:
We target Python 2.7 so that our test suite runs on the broadest set of platforms. This will likely change in the future and 2.7 will be dropped.
We can safely target C++11 because all the default compilers on our supported platforms are ready. Feel free to use C++14 and C++17 provided that all build platforms support this.