How do you set a JavaScript onclick event to a class with css

You can't do it with just CSS, but you can do it with Javascript, and (optionally) jQuery.

If you want to do it without jQuery:

<script>
    window.onload = function() {
        var anchors = document.getElementsByTagName('a');
        for(var i = 0; i < anchors.length; i++) {
            var anchor = anchors[i];
            anchor.onclick = function() {
                alert('ho ho ho');
            }
        }
    }
</script>

And to do it without jQuery, and only on a specific class (ex: hohoho):

<script>
    window.onload = function() {
        var anchors = document.getElementsByTagName('a');
        for(var i = 0; i < anchors.length; i++) {
            var anchor = anchors[i];
            if(/\bhohoho\b/).match(anchor.className)) {
                anchor.onclick = function() {
                    alert('ho ho ho');
                }
            }
        }
    }
</script>

If you are okay with using jQuery, then you can do this for all anchors:

<script>
    $(document).ready(function() {
        $('a').click(function() {
            alert('ho ho ho');
        });
    });
</script>

And this jQuery snippet to only apply it to anchors with a specific class:

<script>
    $(document).ready(function() {
        $('a.hohoho').click(function() {
            alert('ho ho ho');
        });
    });
</script>

You could do it with jQuery.

$('.myClass').click(function() {
  alert('hohoho');
});

You can do this by thinking of it a little bit differently. Detect when the body is clicked (document.body.onclick - i.e. anything on the page) and then check if the element clicked (event.srcElement / e.target) has a class and that that class name is the one you want:

document.body.onclick = function(e) {   //when the document body is clicked
    if (window.event) {
        e = event.srcElement;           //assign the element clicked to e (IE 6-8)
    }
    else {
        e = e.target;                   //assign the element clicked to e
    }

    if (e.className && e.className.indexOf('someclass') != -1) {
        //if the element has a class name, and that is 'someclass' then...
        alert('hohoho');
    }
}

Or a more concise version of the above:

document.body.onclick= function(e){
   e=window.event? event.srcElement: e.target;
   if(e.className && e.className.indexOf('someclass')!=-1)alert('hohoho');
}