I have just beginning having fun with Apache, and that i wanted know.

Basically base myself around the tutorial/good examples I have found, e.g. mod_example.c or mod_hello.c the http appears always to become sent line by line:

ap_rputs("<HTML><HEAD><TITLE>Greetings</TITLE></HEAD></BODY>\n",r);
ap_rputs("<H1>Greetings, Earthling</H1>\n",r);

Could it be a readability problem, or you will find real causes of this ?

It's readability oftentimes.

You can send the whole response (HTML, etc.) with ap_rputs() should you desired to.