JSView is an extension which allows you to view external js/css files.
Well now there's a much easier way. You can use the JSView extension to solve the problem. You can access it from the context menu, from the toolbar, from the view menu, or from the status bar. If the website you are viewing contains any external js/css files, an icon will appear that says "CSS", "JS", or both. Each individual file can then be viewed by clicking on the filename. The file will be opened in a new window. You can also choose to open all external files by clicking on "View All".
With the latest version you can now view the embedded css and js as well. Just open the "Page Info" window from the context menu or tools menu. There will be 3 new tabs in that window. One for scripts, one for stylesheets, and one for frames. You can open them by double clicking or by using the context menu.
The 'view page source' menu item now opens files based on the behavior you choose in the jsview options. This allows you to open the source code of any web page in a new tab or in an external editor.