Once a path-breaking web browser, Opera is now just another Google Chrome clone. I still use the pre-Blink 12.x Presto-based Opera browser and it remains my primary browser of choice even now. I use its Developer Tools utility quite often. It is called Opera DragonFly.
The first time you start DragonFly, it is downloaded from Opera servers and then runs locally. If there is no Internet connection, you will be stuck. As the DragonFly open-source project has been terminated, it is only a matter of time before the servers hosting DragonFly is gone. So, you need to keep a local version of the DragonFly. Get the latest stable version (1.2012.12.14.1) dated 12 December 2012 from:
Then extract its contents locally to a safe folder. Then type “opera:config” in the address bar. Search for “dragonfly” (without quotation marks) and set the URL to the full path of the client-en.xml file from the extracted contents. You can also create a simple text file name AUTHORS so that the About box does not look empty.