LibreOffice Online is a cloud version of the LibreOffice suite. It uses the same underlying engine as the desktop application.

Built from the core code of the LibreOffice project, it allows for the visual display and collaborative editing of documents.
Birth of the LibreOffice Online Project
The development of LibreOffice Online is an HTML5 web-based technology produced by SUSE.
In 2015, this proof of concept was rewritten into an initial development release by Collabora, allowing users to test LibreOffice Online for the first time.
In 2016, the first source code version of LibreOffice Online, a cloud-based office suite that offers basic collaborative document editing in a browser by reusing the "core engine" of LibreOffice, was added to the main branch for the announcement of LibreOffice 5.3.
This latest development brought collaborative editing to LibreOffice Online, a feature that transforms the application into an ultra-modern cloud office suite - the first to natively support the ISO/IEC standard ODF (Open Document Format) with collaborative editing capabilities.
The rendering fidelity of LibreOffice Online is equivalent to that of desktop software, and the interoperability matches that of LibreOffice with support for standard and proprietary document formats.
LibreOffice Online has been primarily developed by Collabora, one of the leading contributors to the LibreOffice codebase and community.