The File API is still kind of poorly supported, but you could add that depending on your browser (though you'd want to use an extension, since it'd be a bit mor

The File API is still kind of poorly supported, but you could add that depending on your browser (though you'd want to use an extension, since it'd be a bit more code). As for live-editing HTML, that's usually been considered out of scope for a web browser, and been pushed to authoring or debugging programs. Chrome ships with their developer console integrated, but IIRC Firefox still relies on extensions.