[philipwalton]

Polymer partially polyfills CSS Custom Properties: https://www.polymer-project.org/1.0/docs/devguide/styling.ht...

Also, for what it's worth, not all new JavaScript features are polyfill-able. Proxies, for example, are impossible to polyfill, should they not have been added? WeakMaps are also only partially polyfill-able, are they overdesigned too?

[underwater]

I believe that where possible it makes sense to follow what is already happening in the wider open source ecosystem, rather than trying to design a solution from scratch.

There is a huge demand for constants/variables in CSS, as well as a widespread use of preprocessors. The CSS variable spec does bring a lot of interesting ideas to the table, but it does so at the expense of ignoring how people are shipping code today.

Designing a solution that can be dropped into an existing pipeline can be valuable even if doesn't introduce any new concepts or features. For example Promises in JS are pretty simple and just replicate the functionality of existing libraries, but they normalized existing patterns and made it possible to trust that third party code would have consistent behavior. That was a huge boost.