Is it OK to stabilize =dev-libs/libebml-1.3.0 ? If so, please CC all arches which have stable keywords for older versions of this package and add STABLEREQ keyword to the bug.
This generally should be stabilized when newer versions of mkvtoolnix are stabilized, not by itself. I'll open a new bug to stabilize things in a week or so when mkvtoolnix-6.2.0 can be stabilized.