Custom_html callback markup is broken

Just looked at the code again and found that you "echo" the content instead of returning it. The callback function "must" return the content.

If your content is complicated, you should use output buffering to return it.

Here it is:

function hello() {
    return '<b style="color:red;">Hello</b>';
}

If you want the callback print the content, you can try output buffering, like this:

function hello() {
     ob_start();
     echo 'Your content';
     return ob_get_clean();
}

This makes the logic of the plugin consistent ('std' vs 'callback').