What I've found helps (at least at the other layers) is to have principles documents and standards documents for the AI to reference when it's modifying code. Principles documents describe the why, and standards documents describe the how.

So for example a few parts from my initial CSS-standards.md (still needs a lot of revision):

    ## Utility-first discipline

    **Raw utilities everywhere by default. Never `@apply` for "components."** `@apply` exists only for
    true low-level primitives that can't live in a template (e.g., `prose` overrides, embedded
    third-party widget shells).

    Wathan's stated position: extract only on "worrisome duplication." The Tailwind team explicitly
    describes `@apply` as a tool you reach for after first reaching for templates. **Premature CSS
    abstraction is the failure mode.**

    ## Spacing

    Use only the default scale (`0, 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, 24…`). **Never `p-[13px]`.** If
    you need a value, change the scale in `@theme`:

    ```css
    @theme {
      --spacing: 0.25rem;
    }
    ```

    v4 uses a single `--spacing` multiplier; everything derives from it.

    ## Anti-patterns (banned)

    - **`!` important prefix** (`!bg-red-500`). Fix specificity properly.
    - **Arbitrary values for colour** (`bg-[#1da1f2]`). Define in `@theme`.
    - **Arbitrary pixel offsets** as default (`top-[3px]`). Use the spacing scale. Tolerated only as
      rare one-offs.
    - **Nested custom CSS more than one level deep.**
    - **`@apply` for any class that wraps fewer than ~5 utilities** or appears in fewer than ~3
      templates.
    - **Dynamic class string interpolation** (`text-${level}-500`) — purger can't see these.
    - **Custom breakpoints in v1.**
    - **Inline `<style>` blocks.** All CSS goes through `assets/css/app.css`.