Imagine if at the top of a stylesheet you could define some variables.

var customGreen = #b3dca3;

Then, whenever you needed that colour in the stylesheet, you could simply write:

.someClass { color: customGreen; }

This would certainly save some time, as you would not have to remember the specific hex value of the colour you require for a certain class. I feel like I spend half my day scrolling up and down my stylesheets to copy and paste existing hex/rgb values from similar classes.

I realise something similar can be achieved using classes like .customGreen {color: #b3dc3a;}, but this would require adding that class to your markup, which is not really semantically ideal. What happens if you want to change your colour scheme to blue? You could change the one reference in the stylesheet and it would work fine, but your markup will have all of these nosensical classes of customGreen that are actually now blue.

What are your thoughts? Maybe this is something that should be available for CSS4. That way I might just be able to use such a feature in about 20 years time – if and when CSS4 is implemented (or even thought of for that matter).