[AlanHK]

In this ePub I used this pair of style definitions:

.letter, .letter1 {
    display: block;
    margin-bottom: 0;
    margin-left: 2em;
    margin-right: 1em;
    margin-top: 0em;
    text-align: left;
    text-indent: 2em;
    }

 .letter1 {
    margin-top: 2em;
    text-indent: 0;
    }

And in the pages used them:

<p class="letter1">Dear Mr. Wells,</p>

  <p class="letter">Your attendance is requested in an investigation
of grave importance to the Royal Family of the Nederlands. Please
consult S. Holmes posthaste regarding your willingness to participate.</p>

  <p class="letter">Sec. to H.M. Emma of Waldeck-Pyrmont</p>

The idea being to have the first line of a letter with a top margin and no indent, otherwise the same as the "body" of the letter.

This looked as expected and validated with epubcheck.
But running Sigil's "Delete Unused Stylesheet Classes" and it includes "letter1" as unused.

Is this a bug in that function, or is this kind of CSS invalid?