Via https://webdesign.tutsplus.com/articles/the-future-of-web-typography-css-fonts-level-4--cms-29911:

"When font-display is omitted in a @font-face rule, the user agent uses the font-display value set via @font-feature-values for the relevant font-family if one is set, otherwise it defaults to font-display: auto."

Spec: https://www.w3.org/TR/css-fonts-4/#font-display-font-feature-values

I don't think this is covered in https://caniuse.com/#feat=font-variant-alternates or https://caniuse.com/#feat=css-font-rendering-controls, as this is like "@font-feature-values Level 4".