If you cant (or don't want to) install any extra app to your desktop OS, just open https://app.pomodoneapp.com/ in your browser, log in and start working.
There are some tips though:
Pin the app's tab. Sometimes browser or special extension kills the tab in background to save your computer resources (CPU and RAM). If the tab is killed, there is no way to continue ticking + there is no way to make sure the timer info will be successfully saved to the cloud log.
Keep in mind, that some hotkeys won't work in browser as long as it can't be overwritten. E.g. ⌘+S / CTRL+S (full sync) will work on the desktop app, but won't work in web app. The same about ⌘+R / CTRL+R (active project sync).
There is no way (at least for now) to define custom sounds in the web app.