Chrome, which has already implemented display:contents behind the flag, is about to ship it: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/XzdNrEvn4Qk/650SpqXTBAAJ
The CSS Display Level 3 spec is ready to transition to CR state (https://www.w3.org/blog/CSS/2017/07/20/display-align-update/), the `display:contents` value is well-defined and stable enough, and there is a good test suite for it.
Please implement this feature, we, web developers, really need it!
WebKit has just enabled display:content by default: https://trac.webkit.org/changeset/224822/webkit
This makes it the 2nd shipped implementation (after Gecko's). Chrome's implementation (behind the flag) is also rather good (it even handles the "unusual" element more-or-less correctly according to the latest spec edition) and probably will be shipped very soon.
So is Microsoft again the only major browser vendor without any implementation of this extremely useful feature? How is it possible?
display:contents has been supported unprefixed in Firefox and is currently in development in Chrome (https://bugs.chromium.org/p/chromium/issues/detail?id=657748). It is also very useful in implementing slots feature from Shadow DOM v.1 spec. And it's very useful for placing elements from different containers into the common grid using CSS Grid Layout (it's already possible in Firefox Nightly/Alpha and likely will soon be possible in Chrome). Is implementing it so hard?
The initial and unset CSS wide values are already in development in Microsoft Edge. The all shorthand property is now on the backlog with a priority of medium. You can track status updates at https://dev.modern.ie/platform/status/cssallshorthand/
Support for using CSS Transforms on SVG elements is now on the Microsoft Edge backlog with a priority of medium.
The :has() pseudo-class is now on the Microsoft Edge backlog. You can track status updates at https://dev.modern.ie/platform/status/cssrelationalpseudoclasshas