base64 code

.ui-menu .ui-menu-item {
	margin: 0;
	padding: 0;
	width: 100%;
	/* support: IE10, see #8844 */
	list-style-image: url(data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7);
}

If you mean this, it’s explained in https://bugs.jqueryui.com/ticket/8844

It is using the data URI scheme.

Thank you for clarifying that. I appreciate your quick response!