Don’t throw when the CSS global is not defined

Author: j-f1

assets/javascripts/app/app.coffee

@@ -252,7 +252,7 @@
         matchMedia:         !!window.matchMedia
         insertAdjacentHTML: !!document.body.insertAdjacentHTML
         defaultPrevented:     document.createEvent('CustomEvent').defaultPrevented is false
-        cssVariables:         CSS.supports and CSS.supports('(--t: 0)')
+        cssVariables:       !!CSS?.supports?('(--t: 0)')
 
       for key, value of features when not value
         Raven.captureMessage "unsupported/#{key}", level: 'info'