Category links in IE 10 stopped working

A new development using IE10 to browse the following site.

http://www.rarecoinsandtokens.co.uk/coins-british-c-81_89.html

The category links in the center panel, which have worked up until now with IE10, no longer work when viewing with IE10.
The text links appear to exhibit a css behavior, but the links are dead.
The category links in the sidebox, and breadcrumbs, work fine.

Everything is ok in Chrome, and Firefox.

Anyone else experienced this problem, or have any pointers to solution, help appreciated thanks.

Zen Cart 1.3.9f

When I press F12 in IE10 and then look at the console, I'm presented with the following errors

HTML1524: Invalid DOCTYPE. The shortest valid doctype is "<!DOCTYPE html>". 
coins-british-c-81_89.html, line 1 character 1
HTML1512: Unmatched end tag. 
coins-british-c-81_89.html, line 17 character 1
HTML1514: Extra "<body>" tag found. Only one "<body>" tag should exist per document. 
coins-british-c-81_89.html, line 19 character 1
SEC7115: :visited and :link styles can only differ by color. Some styles were not applied to :visited. 
coins-british-c-81_89.html
HTML1521: Unexpected "</body>" or end of file. All open elements should be closed before the end of the document. 
coins-british-c-81_89.html, line 296 character 1

and validator.w3.org says:

Line 296, Column 7: end tag for "div" omitted, but OMITTAG NO was specified </body></html>
You may have neglected to close an element, or perhaps you meant to "self-close" an element, that is, ending it with "/>" instead of ">". 

Info Line 21, Column 1: start tag was here <div id="mainWrapper">

Thanks for the reply.
Unfortunately, this does not answer why the site was working until recently.
I am also unable to act upon the results from an html interrogation, as the pages are put together from a composite of php files. Finding a missing tag will be impossible.

This is a major fail in a live shop, and I need to resolve the issue urgently.
Can anyone suggest how I might approach a solution?
Thanks

kevin_a:

...
I am also unable to act upon the results from an html interrogation, as the pages are put together from a composite of php files. Finding a missing tag will be impossible. ...
Badly formed HTML can have a number of potentially negative impacts upon your site. This includes causing links to not function or content to not show up / display correctly. The issues caused can differ by browser, exact browser version (and patch level), the presence of malware protection filters, what device is being used, and other factors. If the template (theme) installed on the website has badly formed HTML (such as unbalanced HTML elements), the best first step to address the issues are to fix any errors in the (generated) HTML.

Doing a search of the file contents inside the "templates" folder (for a phrase near where you want to change the HTML) usually works pretty well to find where the phrase occurs in both HTML and PHP files. If one is not comfortable finding and fixing template (theme) errors, one can go back to the original person who wrote the template or hire someone to fix the errors.

Note: the badly formed HTML does not seem to impact "category links" in my copy of IE10, but it (possibly along with some CSS issues) does cause a large empty space to the right of the screen (and a long horizontal scroll bar).