<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def crop_box(endpoint=None, filename=None):
 """Create a crop box. :param endpoint: The endpoint of view function that serve avatar image file. :param filename: The filename of the image that need to be crop. """

        crop_size = current_app.config['AVATARS_CROP_BASE_WIDTH']

        if endpoint is None or filename is None:
            url = url_for('avatars.static', filename='default/default_l.jpg')
        else:
            url = url_for(endpoint, filename=filename)
        return Markup('<img src="%s" id="crop-box" style="max-width: %dpx; display: block;">' % (url, crop_size))