XBL, a precursor to Web Components in use in Firefox for 15+ years before any ordinary webdev was talking about shadow DOM, was open source in every sense, for example. It was also proprietary.
See also: "Problems with XUL".
> XUL is a proprietary technology developed by Mozilla and only used by Mozilla.
I don't know when that page was written, but there were other developers that were using XUL before it was deprecated. The only one that comes in mind right now is ActiveState and their Komodo Edit/IDE. But there used to be more developers and I recall their being a dedicated page that listed those other products built on XUL.
The documentation of it, and the jump in complexity when it came to XPCOM (which were written in C++), were the reason (in my opinion) why platforms like Electron got popular instead of XUL.
XBL, a precursor to Web Components in use in Firefox for 15+ years before any ordinary webdev was talking about shadow DOM, was open source in every sense, for example. It was also proprietary.
See also: "Problems with XUL".
> XUL is a proprietary technology developed by Mozilla and only used by Mozilla.
<https://mozilla.github.io/firefox-browser-architecture/text/...>