Conversion changes margins in ebook

I have noticed that when converting using calibre it will sometimes change the margin settings e.g. the input will have the following:

.quote {
display: block;
margin: 1em 2em;
}

And the output will be changed to:

.quote {
display: block;
margin-bottom: 1em;
margin-top: 1em
}

I get this when converting from html (zip) to azw3 and when converting the azw3 to epub.

Is there anything I can be doing to preserve the margins?