Attempting to use [CSS variables](https://www.w3.org/TR/css-variables/) as the following ([which are supported by all major browsers](https://caniuse.com/css-variables))
```
:root {
--main-color: #06c;
}
#foo h1 {
color: var(--main-color);
}
```
fails with some very unfriendly error messages:
>* Unrecognized or unsupported property at line 2 character 3.
>* Invalid or unsupported value for property color at line 6 character 10.
Looking at the source code I discovered [this comment](https://gerrit.wikimedia.org/r/plugins/gitiles/css-sanitizer/+/016dc6a6650f0e7f1a0d0b24a8667443a06475af/src/Sanitizer/StylePropertySanitizer.php#24) which says:
>This intentionally doesn't support [cascading variables](https://www.w3.org/TR/css-variables/) since that seems impossible to securely sanitize.
The comment unfortunately does not elaborate further ... I don't know what exactly about CSS variables was deemed impossible to sanitize.