[policy, no patch] Discuss how to update 3rd party CSS libraries while still proving BC for stable themes

Problem/Motivation

In #2886904: display: block for details/summary hides drop arrows in Firefox (normalize.css update) → , we have a situation where normalize.css a third-party CSS library that we use, has a new major version that fixes an important bug in Firefox. The library is included as part of Classy, which means that updating it could disrupt themes extending from Classy. So, the question is how we could update the library as part of the continuous upgrade path, possibly in a way that deprecated the old version.

Proposed resolution

Discuss how we should update the library. Some possibilities:

1. Implement some sort of formal library versioning.
2. Informally add like a normalize7 library and deprecate the old one.
3. Add the updated library to core and rename the old version, but use alter hook (or something along those lines) to alter in the old version for Classy (and possibly stable).
4. Or even have new base themes that like slightly_less_stable or even_more_classy that provide most of the BC that the stable base themes do, except that they track the latest versions of vendor libraries and include CSS and template changes only for bugfixes.

Remaining tasks

• Discuss!
• Track down any existing policy issues related to further evolving our theme layer BC and relate them as well.