Tokenisation and parsing for whitespaces described in CSS Syntax Module Level 3 matches the current WebKit behavior in custom properties.
Some WPTs still have wrong expectations.

Both leading and trailing whitespaces should be stripped:

https://drafts.csswg.org/css-syntax/#consume-declaration

    2. While the next input token is a <whitespace-token>, consume the next input token.
    ...
    7. While the last token in decl’s value is a <whitespace-token>, remove that token."

Middle whitespaces collapse to a single <whitespace-token>:

https://drafts.csswg.org/css-syntax/#consume-token

    whitespace
    Consume as much whitespace as possible. Return a <whitespace-token>.