This property determines the user-agent string for the MSHTML engine. If it is empty, a default user-agent string for Internet Explorer (version 8 or below, depending on the version of Internet Explorer) is used.

This is used for downloading the web page as well as all linked resources, whereas User-Agent set in HttpAdditionalHeaders is used only for downloading the web page. If User-Agent is also set in HttpAdditionalHeaders, the HTTP header takes precedence (for downloading the web page).

None.