mk:
scheme に移行しています。[6] より新しい世代の HTML ベースのヘルプでは mshelp:
URL scheme が使われていて、 mk:
URL
は使われないようです。
[7] INFO: HTML Help URL Protocols ( 版) <https://support.microsoft.com/en-us/kb/235226>
Anything after the double colon (::) must use forward slashes (/), in accordance with URL standards. Normally, backslashes (\) are used only when specifying a file path that is before the double colon (::). However, using forward slashes before the double colon (::) is still allowed.
The following is a list of correct HTML Help URLs:
Filename.chm::/page.htm - Only works inside of HTML Help. Microsoft strongly discourages its use (also known as "super-automagic" URL).
Mk:@MSITStore:filename.chm::/page.htm - The standard HTML Help URL that works with Internet Explorer 3.0 and later (also known as "automagic" URL).
Ms-its:filename.chm::/page.htm - The new standard HTML Help URL that works with Internet Explorer 4.0 and later (also know as "automagic" URL).