Sigil seems to be removing all of the self-contained span tags like:
Code:
<span style="display:inline-block;width:3.75em;"/>
Which destroys the formatting needed to create the effect.
This seems to happen whether HTMLTidy is on or off. Perhaps it's a violation of epub or (x)html specs? Someone else may be able to answer that. Either way... it doesn't look like you'll be able to use Sigil to create a similar effect.
EDIT:
I tried changing all instances of
Code:
<span style="display:inline-block;width:x.xxem;"/>
to
Code:
<span style="display:inline-block;width:x.xxem;"></span>
It makes no difference. Sigil seems to strip empty span tags, regardless.