if your css file contains
Code:
.body {margin-left:5pt; ... etc} (with dot)
it would affect any text in your html marked up with <??? class="body"> e.g.
<p class="body">
<div class="body">
<blockquote class="body">
<body class="body">
If your css file contains
Code:
body {margin-left:5pt; margin-right:5pt; ... etc} (without dot)
it would affect any text in your html marked up with <body ...> e.g
<body>
it would also affect any text in your html marked up with
<body class="xyz"> but with a caveat. If your css also contains
Code:
.xyz {margin-left:10pt}
then, by rules of cascading css, the .xyz would take preference for the left margin. You would get a 10pt left margin and a 5pt right margin
If the css contains
Code:
body.xyz {margin-left:5pt; margin-right:5pt; ... etc}
then only html files with <body class="xyz"> will have the 5pt margins
For your specific epub, it's hard to say more without seeing it. The fact that a typical html file for a chapter only has a plain <body> tag rather than a <body class="calibre
??"> suggests it was not created by a Calibre conversion, but I stand to be corrected on that. I manually add those 5pt L/R margins to non-Calibre epubs all the time. I can't say I've noticed any problems.