display already includes some new values defined in MathML Core:

The display property from CSS Display Module Level 3 is extended with a new inner display type:

Name: display
New values: <display-outside> || [ <display-inside> | math ]

https://w3c.github.io/mathml-core/#new-display-math-value

Name: display
Value: [ <display-outside> || <display-inside> ] | ...

https://drafts.csswg.org/css-display-3/#propdef-display

The New values field value is expected to be joined to the target value space using | as the glue:

[ <display-outside> || <display-inside> ] | ... | <display-outside> || [ <display-inside> | math ]

Would it be acceptable to include math in <display-inside>? The property definition table in MathML Core could then be marked with an informative class, which means w3c/webref would ignore it when scraping definitions from MathML Core.