The use of the MIME content type: "text/html; version=3.0" is recommended to prevent existing HTML 2.0 user agents screwing up by attempting to show 3.0 documents. Tests have shown that the suggested content type will safely cause existing user agents to display the save to file dialog rather than incorrectly displaying the document as if it were HTML 2.0.
To make it easy for servers to distinguish 3.0 documents from 2.0 documents, it is suggested that 3.0 files are saved with the extension ".html3" (or ".ht3" for PCs). Servers can also exploit the accept headers in HTTP requests from HTML user agents, to distinguish whether each client can or cannot support HTML 3.0. This makes it practical for information providers to start providing HTML 3.0 versions of existing documents for newer user agents, without impacting older user agents. It is envisaged that programs will be made available for automatic down conversion of 3.0 to 2.0 documents. This conversion could be carried out in batch mode, or on the fly (with caching for greater efficiency).