System UI font not used on WebKit

[qvignaud]

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

On WebKit the system font is not called by Bootstrap, making some apps using Bootstrap and WebKit looking “weird” and not well-integrated on Linux systems, such as projects built with Tauri.

While this is mostly due to WebKit not supporting system-ui value yet (https://bugs.webkit.org/show_bug.cgi?id=178624), Bootstrap should probably use -webkit-system-ui within the default $font-family-sans-serif list.

As such, here are examples on Linux Mint with WebKitGTK 2.38.6.

Not-integrated display:

Well-integrated display:

Please let me know if I should open a PR as the change seems simple, but may impact many things that I don't even have knowledge of.

Reduced test cases

N/A because not reproducible that way

What operating system(s) are you seeing the problem on?

Linux

What browser(s) are you seeing the problem on?

No response

What version of Bootstrap are you using?

v5.1.3

[mdo]

This was discussed years ago with the very different Linux default font causing unexpected and inconsistent rendering issues. https://fonts.google.com/specimen/Ubuntu is the font I believe, and it's just so different looking. There are some old threads somewhere here that I can't find right now, but changes were made to deliberately not support that font.