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).