[roger64]

Well, this is what I decided to use.
Each verse has its own paragraph style (Chanson) and the stanza is kept within a div (space) tag. As the length of the verses is variable, I could not find a perfect solution and had to compromise. I tried this one on a six inches screen, it should possibly be better on wider ones, and dubious on smartphones.

I made use of a negative indent which seems to be correctly supported for ePub and PDF. A screenshot is joined. No idea for others (hello Hitch!).

p {
  display: block;
  margin-top: 0;
  margin-bottom: 0;
  border: none;
  padding: 0;
  text-indent: 1.5em;
  text-align: justify;
  widows: 2;
  orphans: 2;
}
p.Chanson {
  margin-left: 70%;
  margin-right: 0;
  margin-top: 0;
  margin-bottom: 0;
  border: none;
  padding: 0;
  text-indent: -65%;
  text-align: justify;
  font-style: italic;
}
.space {
  margin-top: 12px;
  margin-bottom: 12px;
}