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!
Save 2 bytes by turning
((this.style[a]=i),this)
into(this.style[a]=i,this)
. See operator precedence.