[Leonatus]

At formatting the <p> in the css of an epub in Sigil, I wished to create a certain distance between single paragraphs. Thus, I added
margin-top: 2em;

O.k. it worked. Only that I found that 2em is a little too much. So I changed the margin-top to 1em, and, surprise: nothing changed, though trying several times. The space between the paragraphs could not be manipulated anymore.

Then, I looked in other epubs; in one of them, I found, instead of margin-top, simply:
margin: 1 0;

I adapted this to my ebook, and, hoops, the margin changed. Could somebody, please, explain to me what has happened, and what has been my mistake?