Problem/Motivation
Firefox 49 is out and has support for <details> which means our polyfill doesn't kick in. Normalize.css sets <details> and <summary> element as display:block which means Firefox won't display the marker, they fixed this in 5.0.0 (https://github.com/necolas/normalize.css/issues/604).
Checkout comparison aligned by @droplet: (removed comments & unchanged rules, re-reordering to match v3)
https://www.drupal.org/files/issues/diff-manual-aligned-by-%40droplet.patch
Here are the change logs for Normalize.css, starting at 4.0.0 (scroll up): https://github.com/necolas/normalize.css/blob/f06565fe8e0e1ad1b88f273271.... There are a lot of changes due to reorganization/sectioning of the code as well as comment changes.
There is one notable change that causes a regression in, at least, Seven: https://github.com/necolas/normalize.css/commit/02c5c7adbbec1707900fde86..., see the attached screenshot.
There may be regressions upgrading from 3.0.3 to the most recent normalize.css.
Proposed resolution
Upgrade core's Normalize to most recent version.
Normalize 3.0.3 should be copied to Stable. Stable needs to continue using 3.0.3 for BC.
Remaining tasks
Replace normalize with 5.0.0- Determine which removed or changed selectors will cause regression.
- Add them back to Stable theme.
User interface changes
Hopefully none!