Some books use paragraphs containing only an nbsp character to provide extra vertical spacing, even though there are better ways to accomplish that. It was the case in the early days of KFX that these paragraphs would be dropped, causing the extra spacing to be lost.
I just did a test with the latest Kindle Previewer (version 3.8) and the following content:
Code:
<body>
<p>First paragraph</p>
<p> </p>
<p> </p>
<p> </p>
<p> </p>
<p>Last paragraph</p>
</body>
The KFX generated is equivalent to only the first and last paragraphs, with the second having a margin-top of 6.65em.
So it does still strip out paragraphs containing only nbsp while retaining the desired vertical spacing in a much cleaner fashion.