mostly's purpose is to serve as a lightweight, easy-to-comprehend starting point, with a focus on providing a great developer experience while helping you get high quality and maintainable web applications deployed rapidly.
It uses Express for the server and React for the client. Worth a look if you're starting up a new web project. I dig this point about it:
Nothing is hidden, nothing is magical, and all of the "plumbing" is accessible and simple.