Re: [csswg-drafts] [css-borders-4] Rendering of outset shadow spread with concave 50% corner-shapes (#13037)

I tested this in Figma and Sketch. By default, their shadows use `miter` joins.

- In Figma, when the default is set to `miter join`, it switches to `bevel join` when a certain threshold is exceeded.
    <img width="812" height="996" alt="Image" src="https://github.com/user-attachments/assets/c71f49b7-3c84-48dd-9824-74f27a94202c" />
- In Sketch, there appears to be no threshold; shadows simply become very long.
    <img width="372" height="822" alt="Image" src="https://github.com/user-attachments/assets/b922c9cc-8b78-42e0-a46a-fa34115970f1" />
- When adding rounded corners, they all become round joins.
    <img width="550" height="770" alt="Image" src="https://github.com/user-attachments/assets/dc32bb58-22d1-4bd2-84be-b298fca15995" />

So, I think we could add a keyword similar to `round` in [`polygon()`](https://drafts.csswg.org/css-shapes-1/#funcdef-basic-shape-polygon) here. Or perhaps a new property: `box-shadow-round: <length>`

When its value is 0, it resembles a `miter join`; when greater than 0, it becomes a `round join`.

## Sketch

https://github.com/user-attachments/assets/ad757b46-80bd-41e0-a66e-49db7b0519be

## Figma

https://github.com/user-attachments/assets/dab2a6dd-867d-4cf9-9381-22c81e0f62ee


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Received on Thursday, 30 October 2025 16:15:55 UTC