Excellent guidelines for writing readable regulations are published by the U.S. Federal Register. It's so nice when government organisations make resources like this available to the general public as well as their own staff.
I hate think of the money wasted as individual agencies laboriously re-invent the ferris wheels of clear communication, when they are freely available online.
The majority of an in-house Style Guide is rightly restricted to staff and contracted writers and designers. They include the cogs of communication: branding guidelines and the debatable minutiae of good writing such as page design, capitalisation of headings, and the use of the Oxford comma (hey there's one!).
But general guidelines like Making Regulations Readable apply to nearly every regulation on earth or online.