Spot on. Any sufficiently complex website won't benefit from this. How would your code editor know which CSS (or even beter, Sass) files apply to which pages? No way.

I'm hoping sourcemaps come to the rescue. We already have this for JS, but the support isn't deep enough. We need support for sourcemaps for templates and css (and even sprites!).

There's some thought been given to multi-level sourcemaps, which are needed for template->js->minified->concatenated... type mappings - see https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiO...

Also there's something on the feature roadmap for firefox to cover css sourcemaps. Not much info, but its a start: https://wiki.mozilla.org/DevTools/Features/CSSSourceMap

If you tried to apply the same thinking to everything in the UI, you'd hit sprites soon enough. It'd be interesting to have enough info that given an (x,y) in an image we can say what the source files were. This is the kind of thing where the http header in that sourcemaps v3 proposal would apply.

+1. We (the core team) have discussed using sourcemaps to support the same "live" browser workflows with preprocessors.