See an example.
Why?I wanted to test the limits of what was possible in the browser from a static site. IceBuddha is hosted on github pages. "Submitted" files are not uploaded anywhere. Everything happens in your browser locally.
Read more about IceBuddha in my post on http://0xdabbad00.com/icebuddha-generic-file-parser/.
Similar projects/products010 editor: Windows & Mac (commercial), odd format for binary templates to parse files, but looks similar to C structs and is often referenced.
Synalize It!: Mac only (commercial); XML based grammar format which means limited capability for more advanced binary file formats.
IceBuddha is mostly abandoned (last update on 2013-10-27 to move the files to github pages). I'm unsure if the concept of a webapp for this a good solution as many things are impossible (ex. saving files). It succeeded in testing the limits of the browser as of 2013.