I\'ve watched an already well known video where Bret Victor, former Apple UI designer shows amazing demos with immediate updates of running code after changing literally one
These days for frontend development a lot of the tools offer something called "hot-reloading", basically making your changes in code immediately visible in the browser/mobile emulator. This is independent of the IDE/editor.
I believe the following allow changing a line in source (and seeing the immediate effects):
Note: I have experience only with 1, 5-7. For many of these, I don't think you can put your cursor on a certain part of the code and have the relevant portions of the image highlighted.
Edit: added items no. 4 (2014.10.27), 5 (2014.10.31), and 6-7 (2015.03.12).
Edit 2 (2015.06.25): from http://sixrevisions.com/tools/code-demo-sites/ (most of these are HTML/CSS/Javascript).
Chris Granger is building something called Light Table that looks like a promising move in this direction. Initially it only support Clojure, but he is promising to support other languages in the future.