A perfect replica (hopefully) of the jQuery .css() method
testNode.css("color","blue") //use it with two string arguments!
testNode.css({color:"red"}) //use it with an object!
testNode.css("color") //make it return the value of a property
It's even chainable!
Credit to @atk for the great minifiing tips!
An annotated version would probably be helpful...
Update: even 1 byte shorter:
function(a,i,n){for(n in''+a!==a&&a)this.style[n]=a[n];return n||i?((this.style[a]=i),this):(this.currentStyle||getComputedStyle(this))[a]}