TOC: Restore missing underlines in Firefox

TOC: Restore missing underlines in Firefox

According to <http://www.w3.org/TR/css-text-decor-3/#line-decoration>,
text decorations are not propagated to the contents of inline blocks
and inline tables, and 'display: table-cell' generates an inline table
when used without any parent table-rows and tables.

Firefox currently seems to be the only browser handling this correctly,
see <https://bugzilla.mozilla.org/show_bug.cgi?id=1222703>.

This is a follow-up to c7894deb.

Bug: T92481
Change-Id: Iae4a1e6fd30be950f0f1c40f47b4d2483002fe72