What about pre-processors (SASS, Less) or templating engines (ERB, Haml, Slim)? That would seem to be a huge task to "un-pre-process" an asset. Going backwards is immensely difficult. These are not easily reversible processes.

Instead, why not make something like IntelliJ/RubyMine/WebStorm/etc.'s Live Edit plugin a native part of the browser. Allow the asset pipeline to tell the browser it has changes available, rather than try to have it regurgitate all the processed assets. That would be relatively easy to add and very straightforward to implement.

Your Live Edit idea is a good one, but I think the assumption about "un-pre-processing" would be part of the pre-processing development tools understanding what was being pushed to them, therefore, the need for an open standard as has been alluded to above.

Love this idea, it would make source modification a piece of cake for every browser-maker to implement. To allow for compatibility, this feature could be activated only when a X-Source-Modification-Enabled (or something) header is present on the original page. The header's value could even be the (relative) endpoint to call!

The simplest middleware that would pop up would be a diff generator. But with source maps, web frameworks could track what exact source files were used.

My crazy idea is to just use git + some sort of source map style overlay on both ends to track changes and pass them back and forth.

A lot of these ideas could be supported via a proxy system. Something like charles http://www.charlesproxy.com/ on steroids. If you wanted to edit yahoo.com , you would checkout your local repo, have charles point to the local repo, and then your on the fly css/js fixes would go to your local repo. Yahoo's server wouldn't have to support it.

It seems to me, that with source maps in the browser, the browser should be able to PUT the original source file, with the correct file hierarchy. So, downloading assets.min.js, editing models/Foo.coffee, PUTting models/Foo.coffee.

That would be really something.

I'd be happy even if this was limited to the local file system! I've already got build and deployment set up, and modifying the web servers seems like a hassle.