What FL Studio actually contributed
The step sequencer existed long before FruityLoops, but FL made pattern-first composition the default way an entire generation learned to produce: build a pattern, clone it, change one thing, drop the patterns onto a playlist. That is a way of thinking rather than a piece of software, and it is why so many producers who have never opened FL still work in patterns. Nothing about it requires a desktop application, which is why it ported to the browser cleanly.
What genuinely does not port
Plugin hosting, offline work, and the deep mixing and automation environment of a mature DAW. Those are not marketing differences - a browser cannot load VST binaries at all, and no amount of engineering changes that. If the finished record is going to be mixed with plugin chains, FL Studio or another desktop DAW is doing that job and a browser studio is at best where the idea started. The technical boundaries are laid out on the browser-based DAW page.
A reasonable way to use both
Sketch in the browser, finish on the desktop. Patterns export as MIDI and stems export as WAV, so an idea caught on a train or a school laptop arrives in FL Studio as usable material rather than as a memory. A lot of people who search "FL Studio online" end up doing exactly this, and it is a better outcome than either tool alone. The genre-specific patterns are on the beat making hub.