[css-variables] Resolving cyclical references in a better way (Issue 3)

At this time, inadvertant cyclical references in CSS will result to poor 
results because it marks every custom property invalid, having for 
consequence the invalidation of many other CSS properties at computation 
time, which mean the ‘initial’ value of the property will be used instead.

A better way to handle cyclical references is proposed in the current CSS 
Custom Properties draft which takes in account the fallback value of the 
broken references to ‘fix’ the issue.