Currently the Fonts 4 draft says that if the optional <angle> is not specified for font-style: oblique, this "represents an angle of "20deg"". However, in my experience the angle used for synthetic-oblique font styling -- which has long been provided by most platform text-drawing APIs, when asked to draw "Italic" text with a font family that only provides Regular -- varies significantly between platforms, with (for example) Windows applying a more pronounced slant than macOS.

In order to better match the "native" text rendering of the platform, therefore, Firefox currently uses a platform-specific value when applying a synthetic-italic/oblique effect. I would suggest that this is appropriate behavior for font-style: oblique when no explicit <angle> is given, and rather than mandating a specific angle in this case, the spec should say something along the lines of "if an <angle> is not specified, the user agent should use a slant angle that harmonizes with the platform's typical text rendering behavior."

Moreover, if we must specify a default here, rather than allowing the browser to follow platform conventions, I think 20deg is too large. Current Firefox behavior -- which was designed to harmonize with host platform behavior -- uses an angle between 11.3deg and 16.7deg. Checking the italicAngle field in a sampling of actual italic/oblique fonts shows values that cluster around 11°-12°, with occasional outliers as small as 6° or as large as 16.3°.

[1] https://drafts.csswg.org/css-fonts-4/#valdef-font-style-oblique-angle