Don't ever put block elements inside inline elements. If you do, in most cases, nothing bad will happen. If you do, watch out. It's better not to do it. Here's what the W3C spec says http://www.w3.org/TR/REC-html40/struct/global.html#block-inline.
Once you have removed the opening SPAN tag (there is no closing tag), I'll have another look.
First off, xtdMenuBuild.js manipulates the document by adding the following inline style rules
When I look at the generated markup I see the following
<!-- (FlexiMenusJS Begin) #Entasis -->
<div style="position: relative;" id="FMJS_Entasis_root"><div style="height:1px;width:1px;overflow:hidden;" id="FMJS_Entasis_rootpreloader"> <br><img src="http://www.entasistours.com/includes/FlexiMenusJS/skins/adobe/images_grayallaround_dropdow n
Also I have noticed that the problem occurs only in IE9 and IE10.
Because I do not know enough about the menu that you have used, I can only suggest that you wait for a more knowledgeable person to come along or go to a website that deals with the used menu system.