Widget results can be returned in List Markup (clean_markup=false) or Clean Markup (clean_markup=true).

In List Markup, widget results are wrapped in div > ul > li with some classes added automatically:

<div id="lw_widget_cache123456789" class="lw_widget lw_widget_type_events lw_widget_events">
        <ul class="lw_widget_results lw_widget_results_events">
                <li class="lw_item_1 lw_has_image">

With Clean Markup enabled, only the HTML you’ve entered into the format settings will be returned (unless you are using randomization, pagination, or added your own classes with other settings, which require additional wrapper markup).