See the question and my original answer on StackOverflow

What Word generates is called "Conditional comments". More specifically, we have here "Downlevel-hidden conditional comments" which take the following form:

<!--[if expression]> HTML <![endif]-->

The expression uses operators and "Version vectors". In general, these vectors refer to "IE" and are used to handle HTML compatiblity issues.

But you can use custom version vectors:

If you develop add-ons, you can use custom version vectors to provide version information to webpages. To define a custom version vector, add a REG_SZ value to the Version Vector Registry key. The name of the new key defines the feature value to use in a conditional comment, as shown here.

         Internet Explorer
            Version Vector
               Contoso = 0.9

The previous example uses a custom version vector to indicate that a pre-release version (0.9) of the fictional Contoso control is installed on a user's computer. This next example shows how a conditional comment might use this information.

<!--[if lt Contoso 2]>
<p>Your version of the Contoso control is out of date; Please update to the latest.</p>

Since VML is itself an (embedded) add-on, you can play with HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Version Vector\VML key and/or the 32-bit one on a 64-bit OS HKEY_LOCAL_MACHINE\Software\WOW6432Node\Microsoft\Internet Explorer\Version Vector\VML. Apparently, completely deleting the key fixed your problem.