Looks like this was regretfully omitted. This apparent mistake has very little impact right now because we almost exclusively embed these variants today.
But, that'll change soon as part of the T121730 experiment (and if positive, would remain that way).
Let's fix this up before we proceed with T121730.
Currently
.oo-ui-icon-bell, .mw-ui-icon-bell:before { background-image: url(/w/load.php?modules=oojs-ui.styles.icons-alerts&image=bell&format=rasterized&skin=vector); background-image: linear-gradient(transparent,transparent),url(data:image/svg+xml,%3Csvg xmlns=%22…/%3E %3C/svg%3E); }