Spec: https://drafts.csswg.org/css-fonts-3/#propdef-font-variant-east-asian Value: normal | [ <east-asian-variant-values> || <east-asian-width-values> || ruby ] <east-asian-variant-values> = [ jis78 | jis83 | jis90 | jis04 | simplified | traditional ] <east-asian-width-values> = [ full-width | proportional-width ] Test case: https://jsfiddle.net/ericwilligers/m6aLhzb3/ The following should each be rejected: 'jis83 jis90' 'full-width proportional-width' 'jis90 proportional-width ruby full-width jis83' WebKit currently serializes these as 'jis90' 'proportional-width' 'jis83 full-width ruby' i.e. it ignores all but the last of the east-asian-variant-values, and all but the last of the east-asian-width-values.