> Even today, web formats cannot render documents with the same fidelity as ODF. Especially not for spreadsheets.

That's a nonsense claim—literally a category error. You're conflating the limitations of what can be represented in a given file format with an implementation's (lack of) support for doing things encoded in that format.

LibreOffice doesn't render documents encoded in plain-text, browser-compatible formats with the same fidelity as ODF because LibreOffice has not implemented encoding documents in plain-text, browser-compatible formats that it renders with the same fidelity as ODF.

There is nothing in ODF that's inherently unrepresentable in JSON or pure XML.