The parts of something that's not it proper

Let's say a web page has a content, style sheets decorating it specifying layout of elements, and a web address where it's located. We say "web page the proper" to refer to the content, but what should I say to refer to its style sheets and web address?

End of XY issues:

I'm writing a specification for a programming language, and so far its language syntax, semantics, and standard library are covered (language proper). So I'm looking for something to describe more or less of its "community culture" stuff, such as spelling of identifiers (Camel case, vs. Snake Cases), and numeric ID assignments, etc. (the core of this question)

Update (further example):

Take microchips as an example: a modern desktop x86 CPU contains several cores, levels of cache, and an integrated interrupt controller (APIC).

When we talk about e.g. "x86 proper", we're talking about its instruction set, but this Q seeks a word that could also describe its satelite parts. E.g. "x86 <.the word I seek.>" to refer to the APIC, and other functional components that weren't originally part of 8086/8087/8088.

I'm not sure you have understood completely what is meant when we add the word "proper" to a noun. It's sense 6 of this definitions page:

strictly limited to a specified thing, place, or idea

So when we talk of a large city and say "the city proper" it means only the parts of the city that are strictly and legally part of it. It doesn't include any suburbs or outer regions that are often thought of as part of the city but are not strictly speaking included. An "x86 proper" is not the instruction set (an abstract concept) but the actual x86 chip. The "web page proper" might include the styling of the page, but possibly not any dialogs or popups that appear when you click on it. The exact nature of "proper" is context dependent - for example a "web page proper" might include the source file for it but not associated style files if you are talking about the source code: but might might mean the entire page (including style) but not associated dialogs if you are talking about the user experience.

For the case you describe, of a programming language, it will be much clearer if you don't look for a word to describe what these elements are not (i.e. "not part of the core language) but instead what they are. You could call them "style guides", "style recommendations", "recommended practices".