1. A computer-implemented method for managing a plurality of specification levels concurrently within a same runtime, the method comprising: executing, on a computer system, a runtime of a middleware, the runtime including an application manager and a set of runtime execution stacks, the application manager configured to establish an interface to the set of runtime execution stacks and the set of runtime execution stacks configured to run independent of one another using representative specification technology runtimes of the plurality of specification levels; receiving a request to start an application having an application manifest; determining, based on the application manifest and the representative specification technology runtimes, a particular runtime execution stack of the set of runtime execution stacks executing on the computer system on which to run the application; and starting the application using the particular application runtime execution stack determined based on the application manifest and the representative specification technology runtimes.