[toast0]

A clean break is hard to manage because of chicken and egg things. If you want to deploy it to your web pages, either you need to ship the required viewer too (and have the user buy in for that), it needs to translate into something today's browsers can use, or you need to do the work both ways.

If your new system is translatable to the old system, it's often not clear what that brings you. And you often end up in the trap that the new system never quite matches the abilities of the old system, so it's hard to abandon the old ways.

[onion2k]

You could do it like picture tags do for images. Define a list of sources inside a new tag, and let the browser pick up the first one it understands with a default old style link tag placed last.