Okay, so this one’s a little old, and I just found it while looking at some of Microsoft Update’s HTML source, it appears to be relevant for MSIE6 and newer and may be responsible for some interesting styling and behaviour of form components.
A quick search for it turns up lots of discussions about other browers such as Firefox being effected if the value is not defined… as such it’s likely a good idea to define it in your pages to be sure.
An old MSDN entry reads…
When running on Windows XP, Internet Explorer 6 and the content displayed in it sports a look and feel that matches the Windows XP platform. You can opt to have your HTML content not take on the same look as the operating system for elements such as buttons and scroll bars, by adding the following META tag:
<meta http-equiv="MSThemeCompatible" content="no" />
Setting this will disable theme support for the document. Some background on this, Windows XP (MSIE6) allows for the use of themes for the operating system to change the general color scheme of many elements.
As such, many HTML components (such as SELECT dropdowns, BUTTONS and INPUT fields ‘MAY’ also be effected if you don’t explicitly prevent it in your code.