mk:

[1] 【M$URI】 HTMLHelp の使っている独自 URI scheme。
[2] HTMLHelp のはじめの、 WinIE3 の世代で導入されましたが、 WinIE 4 世代で新たに導入された mk: scheme に移行しています。
[3] HTML Help URL について <http://www.microsoft.com/JAPAN/developer/library/htmlhelp/SDK/API/1.21/ov_about_html_help_urls.htm>
[4] >>3 にある例: mk:@MSITStore:Helpfile.chm::/Topic.htm[>Window name]
[5] mk:@MSITStore:C:\Program%20Files\Microsoft%20Office%20Beta%20Documentation\Microsoft%20Word%20XML%20Content%20Development%20Kit%20Beta%202\MSXML\xmlsdk5.chm::/htm/xml_concepts_2q9f.htm

[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 (7/8/2015, 3:07:23 PM 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).