docs: cover unspoken rules

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• Links
• System elements

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@WalkQuackBack's "list of things that should probably be clarified":

1. How to handle shadows (e.g. text shadows, box shadows). This seems to be inconsistent across the board.
2. When is it appropriate to use !important vs having a more specific selector for code clarity?
3. Hover, active and focus states. How much should they be lightened or darkened by? Should these be standardized into the library? In theory, tints and shades should help with this, but a solution should be had in the meantime.
4. Faded backgrounds such as diff code added/removed. Should the fade or mix amount on this be standardized?
5. Application of mixin: way it should be applied. As an example, some userstyles do body:not(.dark) and body.dark or [data-theme=“light”] and [data-theme=“dark”] while others do body and body.dark or mroot and &[data-theme=“dark”].
6. Encourage using mixins for repetitive code
7. Specify how apps with colour ramps should be bound to Catppuccin. Atp, this is all over the place.
8. How should semi-transparent backgrounds be handled?
9. Userstyles should prefer using attribute selectors over generated classes. I am personally not a fan of using localization specific selectors though.
10. Usage of vendor prefixes and experimental/not baseline features should be mentioned.

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Related: #2046.