[Phssthpok]

I've just found another example. A chapter heading and subheading use these rules in the original EPUB:

.h2 {
  font-family: serif;
  text-align: center;
  text-indent: 0;
  margin-top: 3em;
  margin-left: 0;
  margin-right: 0;
  margin-bottom: 3em;
  font-weight: normal;
}
.h2a {
  font-family: serif;
  font-size: x-large;
  text-align: center;
  text-indent: 0;
  margin-top: 0;
  margin-left: 0;
  margin-right: 0;
  margin-bottom: 2em;
  font-weight: bold;
}

After conversion, what I end up with in the AZW3 is this:

.h2 {
    display: block;
    font-weight: bold;
    text-align: center;
    text-indent: 0;
    margin: 2em 0 1em
    }
.h2a {
    display: block;
    font-family: serif;
    font-size: 17pt;
    font-weight: bold;
    line-height: 1.2;
    text-align: center;
    text-indent: 0;
    margin: 0 0 2em
    }

So the main heading (<h2 class="h2">) ends up with the default H2 font size, but the subheading is reduced to 17pt, which looks tiny on my Kindle (about 50% of the size of the text in the chapter). In the Calibre editor, it looks quite normal: 17pt = 23px. The chapter text is 1em = 18px.

The margins are also changed in the main heading from 3em top and bottom to 2em top and 1em bottom.

It seems that all font sizes are being expressed in points, and 1em is being treated as equivalent to 12pt, and Kindle points are nothing like 1/72in. And it's also got its own ideas about what margins I should be allowed to have.