I tried to use css display: flex in the Connect Designer but it don’t work. In the preview mode it doesn’t show the expected result and in the source mode my element has display: inline-block.
Will Flexbox be supported in a future release of Connect?
Your observation is correct. Flexbox is currently not supported within the Designer. One can use the property for Web pages as these are rendered by the (external) browser of the visitor (but still you will not be able to preview this in the Designer).
We are currently working on an update of the internal browser. This update includes the Flexbox property. It is most likely to appear in the version after the oncoming 1.7 release.