In MSHTML the interfaces are often numbered. The numbers are at the end of the interface names like IHTML{name}{number}. Interfaces like:
IHTMLRuleStyleIHTMLRuleStyle2- …
IHTMLRuleStyle6
or
IHTMLElementIHTMLElement2- …
IHTMLElement7
A list can be seen in this Microsoft page
Thanks @Jay for your comments.
The numbers represent the version of MSHTML and the version numbers have an indirect relationship with the version of Internet Explorer, CSS, HTML, etc…
I wish I had did this search before but Geoff Chappell’s site has a great chart mapping versions of MSHTML to IE and Windows.
Judging from the chart, the version of MSHTML is closely related to the version of Internet Explorer.