In the past, a valid keyword for viewport value was "default". This would become the webkit client's default value for the given viewport key, similar to "auto" when the value is undefined. I don't think this concept is mentioned in the "css-viewport" spec, but this is a regression from former behavior and functionality: http://people.opera.com/rune/TR/css-viewport/ I'm still trying to work out whether or not it needs a new "ValueDefault" enum value, or if "default" is just a way to explicitly say "ValueAuto". Also, is this even necessary anymore? It seems as if web content wants to dynamically change its viewport, it should just set the <meta> tag again, and that will entirely replace the old viewport settings.