Well, there are no much elements on the widget, as it follows wordpress widgets standards.
<li id="sideposts" class="widget widget_sideposts"> which holds the entire widget.
<h2 class="widgettitle"> the wordpress standard for widget titles.
And then you have an unordered list with all widget posts in the form:
<li>, the post title,
<span class="sideposts_date"> for the date,
<img>for the excerpt thumbnail and a
<p> tag for the post body.
With that I made all kinds of customization, think no more is needed, as only are three different elements: Title, Date, and Body. And already different tags are used for it.
But as always I'm open to suggestions, so tell me what and where you want classes added. If you think it will be more useful, I will add classes around all this elements in the next version.