classId

Classid is an HTML attribute used within the object element to identify a COM class (the CLSID) that should be instantiated as an ActiveX control on Windows browsers. The value is typically prefixed with "clsid:" followed by a GUID in the form xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx. When a browser that supports ActiveX encounters an object tag with a classid, it loads the associated COM component and creates an instance via the operating system’s COM infrastructure. A codebase attribute may be used to specify a location from which the control can be downloaded, and other standard object attributes (such as width and height) may accompany it.

Usage and scope: Classid was primarily used to embed ActiveX controls in web pages for Internet Explorer and compatible environments. It ties the page to Windows-specific components, limiting cross-platform compatibility and tying functionality to the presence of the appropriate ActiveX control on the client machine.

Security and deprecation: ActiveX and the classid mechanism have been associated with significant security risks, including the execution of arbitrary code. Modern browsers generally do not support ActiveX, and the object element with a classid is blocked or deprecated in most contexts. Enterprises that require legacy functionality may use Internet Explorer mode in some modern browsers, but widespread support is no longer expected.

See also: CLSID, COM, ActiveX, object element.