I'm to implement a theme in wordpress.
Is possible modify a output (html) of widget in wordpress?
For example, the "recent comments widget"?
I would copy the widget from the core as needed, put it in the theme or plugin, but you should also unregister the core widget you are replacing.
That can be done like this:
Then register your new widget and you should be all set.
Yes, you can modify the output of the widget, but not by modifying core files. What I would do is, go into the wp-includes/default-widgets.php and copy the entire widget code and rewrite it as you need it. I would also then paste the following into your functions.php file. That way the WP default one doesn't show up in the admin. Also, make sure to use a different class name for the new widget.