Add note for SVG as border-image

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/border-image-slice

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

If an SVG with a `viewvBox` parameter is specified as border-image unexpected/undesirable scaling might occur.

### What did you expect to see?

A warning/note regarding SVG with a specified `viewBox`

### Do you have any supporting links, references, or citations?

with an SVG with a viewbox: https://codepen.io/theking2/pen/NPrmWdW
with the same SVG without viewbox but: https://codepen.io/theking2/pen/JoKzQNp

### Do you have anything more you want to share?

Behaviour might be browser dependent. My tests were on 147.0.3 (64-bit) Fedora Firefox.

### MDN metadata

Page report details

* Folder: `en-us/web/css/reference/properties/border-image-slice`
* MDN URL: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/border-image-slice
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/css/reference/properties/border-image-slice/index.md
* Last commit: https://github.com/mdn/content/commit/1dbba9f7a2c2e35c6e01e8a63159e2aac64b601b
* Document last modified: 2025-12-05T16:09:35.000Z