Bit of random positive feedback, but my home internet randomly died today, and I remember often reading about people being unable to use C3 offline. However, my experience was successful!
I use Chrome with no extensions (My general browser is Firefox, but I use Chrome strictly for C3).
I hadn't opened C3 today, but when internet died, I immediately opened Chrome, and I have a home tab that opens editor.construct.net/beta . I noticed it was loading for longer, so I anxiously waited, and after about 10 seconds, I suddenly see C3's start page. Very good!
I opened a project and saw a popup stating that I'm using free version, which was a worry, but then after a further 10 seconds, it suddenly logged me in with a little "crossed-out cloud" icon, and I was all set. Previews work fine.
This was a pleasant surprise! I know positive feedback is less common to see, as it makes sense to post to highlight issues or get support, but I thought I'd share my experience.