attr('onclick') must use capital C?

attr('onclick') must use capital C?

In this code:

  1. .attr('onblur', 'alert("test");')
    .attr('onfocus', 'alert("test");')
    .attr('onclick', 'alert("test");')
the last one will not work. I discovered that 'onClick' must be used.
Why is that?
W3C for example lists the "onclick" event written with all lower case letters.

I use (an older) version of jQuery 1.5.2 and it behaves the same on IE and FF.