Just look at the example below:

a {
color:yellow; font-weight:normal;
}

It looks like normal code and it seems there is nothing to optimise, isn’t it? Actually this code can be optimized by 20.9% with CSS Optimizer. The new code looks like this (and that is standart compression type):

a {
color:#FF0;
font-weight:400;
}

But you could go further and choose the highest compression rate which will output:

a{color:#FF0;font-weight:400;}

and that is a style 30.2% smaller than the original.
This tool has many options to ensure that the new style will be the closest to your needs.
To optimize your CSS code follow this link.