counter-increment is supported by Aspose.Words however there looks to be some issue with the HTML. If you open this HTML in MS Word, you will see the same output as Aspose.Words so Aspose.Words mimics the behavior of MS Word in this case.
Looks like browser is resetting the counter for body element but Aspose.Words is resetting it for each ordered list (ol element). MS Word produces the same output in this case.
This issue has been logged into our issue tracking system as WORDSNET-12219. We will keep you updated on this issue in this thread.