'Unknown' entries in the Ubuntu Dock when playing video

When playing videos in Web, two Unknown entries (or more, if you close the video and open it again without closing the tab) come up in the Ubuntu Dock (in Ubuntu 17.10's GNOME session). See, e.g., the video on https://en.wikipedia.org/wiki/Satoshi_Tajiri (if you go to the link of the video, rather than the overlay, then only one Unknown entry appears and it disappears when you pause the video). See also twitch.tv videos.

Neither Chromium 62 nor Firefox 57 have this bug.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: epiphany-browser 3.26.1-1ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-17.20-generic 4.13.8
Uname: Linux 4.13.0-17-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.20.7-0ubuntu3.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Dec 7 21:14:25 2017
InstallationDate: Installed on 2017-08-03 (126 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: epiphany-browser
UpgradeStatus: Upgraded to artful on 2017-10-21 (47 days ago)