It is indeed. Mozilla's WebThings Gateway does everything locally, by default. You and your home are the center of its universe. The add-on system not only enables lots of smart home device interop, but also lets you bring in other web content from the Internet that you might want to tie into your smart home. I pull from USGS for earthquakes >5.0 and within 400km of my lat/lon. I also use my lat/lon for time/date rules, local tide charts, and local weather. A rule tells my (always on mute) Google Home speaker to announce "An earthquake >5.0..." when such an event comes in. I also love the (local) voice add-on. It uses Snips wakeword and speech-to-text and a customized-by-Mozilla intent parser and interface to the web thing API so that when you create a new thing or change a name, the local language model is updated immediately. Works on RPi3 very well. No Internet required. The Snips part of the install is currently a hack though, and broke in 0.9. If you installed using 0.7 or 0.8 it will still work. But otherwise you'll have to wait for the 0.9 installer fix.

For tech people, this is easy to use. Still some UX/UI updates needed for mainstream consumer readiness. Remaining big problem is that smart home devices don't tell you whether or not they are web of things ready (direct or via an addon). Need to check the wiki or ask online.