Adding improper CSS Rule

[greench]

Opera Dragonfly cannot allow developers to add incorrect css properties or values that are not recognized by Presto. That should be done properly. However, developers, mostly novice developers, may make typo errors when adding a css property or values. It isn't determined by Opera's core and all typed letters disappear.

Example Case:

1. Write a css rule into an element.
   
2. It cannot recognized by Presto 2.10
3. It seems to be added but it is not affect the element
   
4. If you want to edit the rule, you double click the rule.
5. It gone. There is nothing related with background-image property.

If there exists an over lined CSS rule, it would be more accessible and manageable for developers.
Mockup: