Hi!

  zupsko\l'-\w'zupsko'u\(ul'
is rendered as 
  zupsko______________________________________________________________________________
(i.e. to the right margin) by all mandocs I have in -Tutf8
and as absolutely nothing in -Thtml.

Conversely, groff renders it as underlined zupsko in all modes.

The mandoc output is the same if the width is -3n;
again, groff renders it as zups_k_o_.

More worryingly,
  zupsko\l'|0\(ul'
does nothing on mandoc, instead of correctly underlining the whole line.

In the final case I'm happy to say that mandoc is straight-up broken,
since that /exact/ spelling I encountered in 4BSD tty(4), which uses
  .de Ul
  .ie t \\$1\l'|0\(ul'
  .el \fI\\$1\fP
  ..
to delineate subheadings (McKusick's The CSRG Archives CD#1,
4.0/usr/man/man4/tty.4)
(an equivalent macro appears in the troff user's manual;
 ibid., 3bsd/usr/doc/troff/m3,
 but not in the pages themselves).

The same appears in fsck(8) and and at increasing frequency in manuals
up to 4.3BSD; \l with negative (leftward) distance doesn't seem to've
been used.

Best,
наб