CodeSandbox does almost everything in the browser however. The only thing it doesn't do in the browser is dependency bundling, which StackBlitz does in this case.

Essentially CodeSandbox can work offline if you already had the website open while you're online, then you can still continue offline. StackBlitz has the advantage that the tab where your application is can also be opened initially offline, which is handy for showing your application when you have no connection, but if I'm not wrong you cannot continue developing offline. I'd love to hear that I'm wrong, I'm really impressed with StackBlitz.

I'm the author of CodeSandbox btw :).