Remove default values from shorthand declarations

[GoalSmashers]

a{background:transparent url(...) 0 0 / auto repeat scroll}

into

a{background:url(...)}

Same applies to font, animation, and transition.

[Venemo]

What happens if all the values of a shorthand are defaults?
For example,

foo{background:transparent none repeat 0 0 scroll}

Is this the shortest possible form of it?

foo{background:none}

Or is there something even shorter that's still valid?

[Venemo]

Okay, maybe this is even shorter and still valid:

foo{background:0 0}

But according to W3C this is NOT equivalent to

foo{background:0}

Because when only one unit is specified for background-position the second value is treated as 50%.

[GoalSmashers]

It indeed should be :0 0 - please see https://github.com/GoalSmashers/clean-css/blob/master/lib/clean.js#L241

[Venemo]

@GoalSmashers I've just noticed something: seems that for whatever reason, the property optimizer is not called when there's only one property - which means it can't perform this optimalization when there's only one property. I'm now looking into fixing this. Do you have an idea?

[GoalSmashers]

@Venemo Because of https://github.com/Venemo/clean-css/blob/master/lib/properties/optimizer.js#L116 ?

[Venemo]

@GoalSmashers Yes, that's it! Thanks!

[GoalSmashers]

Fixed with #249.