If you change the /preview to /edit you can then go to File→"Make a copy", and you'll have your own editable version of this document. If you change that one to /preview again, you (unsurprisingly) get something that doesn't use canvas, but uses the DOM with the usual div/span soup. Here's an example I created: https://docs.google.com/document/d/1RnQonRlivBogXxSl1nTZSCfR...
These two /preview versions would be a good basis for comparison (until we have examples of editable documents that use canvas-based rendering).
Visually, comparing the two /preview documents by alternating rapidly back and forth between the two tabs shows some minute differences in kerning (on only some of the lines). It might be interesting to repeat this experiment on multiple devices and see whether and in what way the differences themselves differ across devices.
One difference I've been able to find is that using the browser's in-built find doesn't work. But all the common ways of triggering search (Ctrl-F or Cmd-F, etc) trigger the search function of Docs, and that seems to work identically in both versions. And anyway the browser's find cannot find things off-page before scrolling down the page, so it's not usable in general anyway.