font-variant-east-asian accepts invalid values

[Eric Willigers]

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.

[Brent Fulgham]

Safari, Chrome, and Firefox all agree on rendering for this test case. I don't believe there is any remaining compatibility issue.