The Twine Specs repository on GitHub was updated today to include two new files:
As of this writing, Twine Utils supports reading Twine 1 HTML and TWS, Extwee (my own tool) will support both in the upcoming 2.2 version, and Tweego has had support for Twine 1 HTML for many years with TWS support being considered.
Between the documentation and tool support, authors/developers should be able to hopefully create new tools, functionality, or otherwise understand these historical output formats used primarily from 2009 to 2014 with Twine 1.
(Historical Note: any HTML output created between 2006, the year the twee command-line tool was introduced, and 2009, when Twine 1.0 was published, should match the Twine 1 HTML documentation, but I struggled to find any HTML examples from between 2006 to 2009 to verify. If anyone can send or link me to a HTML file created with twee before 2009, please let me know!)