From what I understand, Collabora basically runs LibreOffice on the server side and relays the resulting image to the client. This looks like it has similar approach. And I'm sad to say it but Collabora is near unusable compared to Google Docs.
Humanity really does need an online self-hosted office suite that natively uses OpenDocument format, but I'm afraid that to achieve that it'll be easier to start from scratch with browser version as a first platform in mind.
Collabora office uses a custom web ui for controls, dialogs, etc. The document body is rendered by the server and streamed in raster tiles over a web socket.
Collabora is perfectly fine, what’s your gripe with it?
It is not perfectly fine. It is dreadfully slow and nearly unusable compared to Google Documents or native LibreOffice.