• [featureVars] Allow to register the same set of substitution rules to multiple features. The addFeatureVariations function can now take a list of featureTags; similarly, the lib key 'com.github.fonttools.varLib.featureVarsFeatureTag' can now take a comma-separateed string of feature tags (e.g. "salt,ss01") instead of a single tag (#3360).
• [featureVars] Don't overwrite GSUB FeatureVariations, but append new records to it for features which are not already there. But raise VarLibError if the feature tag already has feature variations associated with it (#3363).
• [varLib] Added addGSUBFeatureVariations function to add GSUB Feature Variations to an existing variable font from rules defined in a DesignSpace document (#3362).
• [varLib.interpolatable] Various bugfixes and rendering improvements. In particular, a new test for "underweight" glyphs. The new test reports quite a few false-positives though. Please send feedback.