x11-misc/lightdm-kde has not seen any development since 2013 and consequently still rdepends on kde-apps/plasma-runtime, which in turn blocks dev-qt/qtwebkit:4 removal. CCing hwoarang since x11-misc/lightdm[kde] would need to be dropped.
commit f3674be4b8e9ef317c5059170a79a2f3a0902f50 Author: Michał Górny <mgorny@gentoo.org> AuthorDate: Wed Jul 5 11:52:09 2017 Commit: Michał Górny <mgorny@gentoo.org> CommitDate: Wed Jul 5 12:00:08 2017 x11-misc/lightdm-kde: Remove last-rited pkg, #620796