I just converted your HTML code to PRC using Mobipocket Creator and then back to HTML using mobi2oeb.
Code:
<html><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style type="text/css">
blockquote { margin: 0em 0em 0em 1.25em; text-align: justify; }
p { margin: 0em; text-align: justify; }
</style>
</head><body><p>J’ai une excuse sérieuse :</p>
<p>J'ai une excuse sérieuse :</p>
<p>J'ai une excuse sérieuse :</p> <br style="page-break-after:always" /></body></html>
That is the resulting code I get back from the PRC.