We currently support 'display: contents' for slot elements (see https://bugs.webkit.org/show_bug.cgi?id=149439 for details). We should expand this to be generally usable for other elements.
I believe this bug should have higher priority since the current implementation results in wrong behavior: '@supports(display:contents)' feature request claims that browser supports this value (which is partially true since it can parse it), but it doesn't apply actually. Tested in iOS 10.3 Safari.
Enabled by default in https://trac.webkit.org/changeset/224822