Thanks to AJAX, web development became more like desktop application development. Nowadays it is not possible to think computing environment without networking, and network speed is getting faster for non-intranet networking.
Eventually IDEs for developing web application is based on the web browser.
The Mozilla announced such an IDE called Bespin.
I don’t know how it will handle network disconnection case, but if they utilize a cache file of a web browser, there will be no problem.
Here are two screenshots of the Bespin.