Some stylesheets are generated by computers such as when you feed a Word .docx file into Vellum or use InDesign to create an ePub. Then there are the corporate stylesheets used by many publishers. All great sources for unnecessary complexity.
You may want to check
HTML and CSS Tags Supported in Kindle Format 8 though those lists are incomplete.
If you open the azw3 ebook in calibre's ebook-editor and enable the Live CSS tool, it helps to see what is being applied.