Created attachment 101012 [details] Demo of current behavior See §7.3.2. Multi-column Layout in Orthogonal Flows of CSS3 Writing Modes: http://dev.w3.org/csswg/css3-writing-modes/#orthogonal-multicol
<rdar://problem/9783787>
1- The description of what the test should lay out (expected results) is not clear, is not explicit: "This document should not be T-shaped". 2- FWIW, I get the same layout in both Firefox 54 and Chrome 60, once I add unprefixed property names: |¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯| | Containing flow | | | ----------------------------------- | | | | | | | | | | | | | | | | | | | | | | | | ----------------------------------- | | | Containing flow | | | ----------------------- 3- The original section regarding multi-column element in orthogonal flow "7.3.2 Auto-sizing Block Containers in Orthogonal Flows" was at-risk https://www.w3.org/TR/css-writing-modes-3/#auto-multicol and is no longer in the Editor's draft: " Deferred the automatic multi-column behavior of orthogonal flows to Level 4. " https://drafts.csswg.org/css-writing-modes-3/#changes-201512 4- Therefore, I think P2 is not justified.
I revisited this bug report and now understand what "This document should not be T-shaped" means, refers to. I have developed related [multi-column containers in orthogonal flow] tests in http://www.gtalbot.org/BrowserBugsSection/CSS4WritingModes/ and there are a few others in orthogonal multi-column with a vertical writing-mode computes height to fill page rather than wrap contents https://bugzilla.mozilla.org/show_bug.cgi?id=1269152 and I hope to develop more tests later.