Some fonts (e.g. media-libs/noto-emoji) requires freetype with png support enabled in order to display properly in web browsers such as chromium. Adding DEPEND="media-libs/freetype[png]:=" to the chromium ebuild should resolve this issue.
See https://bugs.gentoo.org/753569 for why this was not added in the noto-emoji ebuild.
This applies to about anything that can use those fonts.
e.g. if I disable freetype[png] I can't see noto-emoji in xfce4-terminal anymore
But does that mean xfce4-terminal (or its deps used to render fonts), need to depend on freetype[png] now? For an optional font people may not be using?
I see this as the equivalent of disabling USE=jpeg on a package and wondering why jpeg images aren't showing anymore. USE=png isn't a desktop profile default for nothing.
Not that it's up to me, maybe maintainer will think otherwise.
This can't be solved by adding a USE dependency on media-libs/freetype[png], because chromium does not require it to build (or possibly any other application). I think the right approach here is to add an elog to media-libs/noto-emoji informing users to install media-libs/freetype with png support to work correctly in other applications (web browsers, terminals, ...). I therefore suggest to reopen bug #753569.