Is this a custom skin or the default skin?

If custom; you may want to try your site layout in the HTML 5 doctype:

Source: http://www.falconwebtech.com/post/200...

So, from there my quandry was how to set the DOCTYPE in a DNN Portal. Well, In 4.4 and above, you are able to define the xhtml mode as a part of the skin by adding an xml file that that is named [SKINNAME].doctype.xml. Within this xml file you should declare one node SkinDocType. Then, within a CDATA tag, you add the doctype declaration to be used for this skin. For example, if your page uses Portal.ascx as the skin, you would create Portal.doctype.xml in the same directory. Be careful though, changing the DOCTYPE might have nasty side effects on some skins. The xml file should have the following content:

<!--[CDATA[<?xml version="1.0" encoding="UTF-8"?-->]]>