Sticking to the spec is one way to be portable. Building a portability layer is another one; this is how Common Lisp grows, by taking advantage of the work of different implementations and using standardized tools (FEATURES, reader conditionals) to build a common interface. See e.g. CFFI which has become the (defacto) standard for loading C libraries (https://common-lisp.net/project/cffi/spec/cffi-sys-spec.html...).