Ask HN: Do you know of a browser based DSL development tool?

I don't know where I found it, and I cannot find the link in my history. I'm dumbfounded that I didn't submit it to HN.

The tool had a two pane interface, one for the grammar and one for the language you were targeting. I remember that there were multiple interesting programming tools (one for music?!) implemented in the browser.

I have no clue what the name is, and it doesn't show up in search engines under obvious-to-me search terms.