In 6.9.1. Basic syntax (of @font-feature-values):

Each declaration’s value must match the grammar <integer [0,∞]>+, or else the declaration is invalid and must be ignored.

Then 6.9.2. Multi-valued feature value definitions defines restrictions for declaration values, but only in @character-variant:

For <@character-variant>, [...]. If more than two values are assigned to a given name, a syntax error occurs and the entire feature value definition is ignored.

However, Chrome and FF also seem to restrict declaration values in @annotation, @ornaments , @stylistic, @swash, to a single <integer>. They do not seem to support @historical-forms.

Tests: Codepen (please note that FF does not support CSSFontFeatureValuesMap).