By default, jsFiddle puts all of your code within an onload
handler, like this:
window.onload = function() {
// ...your code here...
};
That means your alertme
function isn't a global, and it has to be for onclick
attributes to work.
There's a drop-down on the left, near the top, that controls this behavior. Change it from onLoad
to one of the "no wrap" options to make your function global.