docs: added new entries to BrowserSupport table (#4633)

* docs: added entry for `attr()`

* chore: slight optimization

* docs: added another feature

* chore: slight optimization

* Update BrowserSupport.md

* Update BrowserSupport.md

Author: mfranzke

packages/foundations/docs/BrowserSupport.md

@@ -11,10 +11,12 @@ This may leave some older browser versions behind. If you need to take care of t
 | [CSS `field-sizing: content` property](https://caniuse.com/mdn-css_properties_field-sizing_content) | 123 (19.03.2024) | tbd             | Technology Preview (220)                                                                                             |
 | [Customizable select elements](https://caniuse.com/selectlist) (planned)                            | 134 (19.03.2024) | tbd             | tbd                                                                                                                  |
 | [Anchor Positioning](https://caniuse.com/css-anchor-positioning) (planned)                          | 125 (14.05.2024) | tbd             | ["this fall"](https://webkit.org/blog/16993/news-from-wwdc25-web-technology-coming-this-fall-in-safari-26-beta/#css) |
+| [CSS Custom Functions](https://www.bram.us/2025/02/09/css-custom-functions-teaser/) (planned) | 139 (30.07.2025) | tbd | tbd |
 
 ## Without graceful degradation
 
 | Web Feature                                                                                             | Google Chrome    | Mozilla Firefox  | Apple Safari      |
 | ------------------------------------------------------------------------------------------------------- | ---------------- | ---------------- | ----------------- |
 | [`@property` / typed CSS Custom Properties](https://developer.mozilla.org/en-US/docs/Web/CSS/@property) | 85 (25.08.2022)  | 128 (09.07.2024) | 16.4 (27.03.2023) |
 | [`light-dark()`](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/light-dark)               | 123 (19.03.2024) | 120 (21.11.2023) | 17.5 (13.05.2024) |
+| [`attr()` CSS Values 5 extensions](https://developer.mozilla.org/en-US/docs/Web/CSS/attr)                             | 133 (04.02.2025) | tbd              | tbd               |