I'm really afraid that this is preparing the final drop of browser support. (We've seen similar in http, where many of the http/1.1 (1997) features, like multipart-http, for-headers, etc. aren't supported by any client for years by now.)
As for MS-Word and HTML: When I finished my thesis in the mid-1990s, I saved it both in the MS-Word version, I used to write it (MS Word 5 for Mac), and HTML (expecting future compatibility). I can still open the Word version, but I may be soon unable to conjure a formatted display of the HTML-version. And I can still display a PDF 1.x...
As for MS-Word and HTML: When I finished my thesis in the mid-1990s, I saved it both in the MS-Word version, I used to write it (MS Word 5 for Mac), and HTML (expecting future compatibility). I can still open the Word version, but I may be soon unable to conjure a formatted display of the HTML-version. And I can still display a PDF 1.x...