Has anyone checked out OnlyOffice?
Is this an alternative to LibreOffice? The GUI seems limited at first look.

Does it support ODT files well?
More specifically, I wanted to ask:
LibreOffice Writer has View -> Web

I like seeing a continuous page.
Does OnlyOffice have that feature?

AFAIK only-office, libre-office, and open-office all originated the same place (possibly open-office) and were forked for different development paths. It seems libre-office became the standard for most linux distros.

There are and will be differences. You choose whichever you prefer to use.

If one wants specific features that do not exist at present one should consider contacting the developers directly and asking for those features. This includes the gui as well as other features.

Assuming vmelkton is referring to this OnlyOffice, it's an independent browser-based office suite, formerly called TeamLab Office, and is entirely unrelated to those other office suites.

OpenOffice.org was a Sun project, started in 2000 from the source code of Sun's proprietary StarOffice.
LibreOffice was forked from OpenOffice.org after Oracle bought Sun (because people rightly distrust Oracle) and is maintained by The Document Foundation.
Subsequently the OpenOffice.org code/rights was handed over to Apache, who now maintain Apache OpenOffice.
(There's also a Mac-only NeoOffice, originally forked from OpenOffice.org, now based on LibreOffice, plus a couple of other forks which are no longer active).

There is a browser-based version of LibreOffice, which is called LibreOffice Online (but isn't hosted anywhere).
Collabora provide paid "enterprise" rebranded versions of both LibreOffice and LibreOffice Online (but you need to register for access to the demo).

Yes, that is their website.