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I find in .css there are two section to control the real margin. The first one is mentioned above, it is body section to control whole margin and it is better to set to zero. The second is the detail text section in paragraph, it is better to set to a very small value. To process CSS like this, the "f" and "j" issue will be disappeared.
Example: this code is in html of kepub
Code:
<body>
<p> </p>
</body>
The
body section in .css will be added with
margin: 0;. The
p section in .css will be modified like this:
margin: 0.25em;
However, I don't find any setting in Calibre can modified this automatically.