<poem> foo : bar :: baz </poem>
returns
<div class="poem"> <p>foo<br /> <span class="mw-poem-indented" style="display: inline-block; margin-left: 1em;">bar</span><br /> <span class="mw-poem-indented" style="display: inline-block; margin-left: 2em;">baz</span></p> </div>
but should return something like
<div class="poem"> <p>foo<br /> <span class="mw-poem-indented mw-poem-indented-1">bar</span><br /> <span class="mw-poem-indented mw-poem-indented-2">baz</span></p> </div>
I am pretty sure that there will be no need to define more than 9 indentation classes (aka 10 different indentations of text including the zero).
Hardcoding of inline stylesheets breaks accessibility and disables custom overriding (eg. wider gap).