Ciao, grazie per la risposta.
Sono sempre stato abituato ad usare i cms ed ora mi sto cimendando in realizzazioni "fatte a mano".
Premettendo che tutti gli altri contenuti che ho inserito nelle pagine, dal testo alle foto, vengono riadattate perfettamente, pubblico il css:

codice:
/* Box-sizing -*/
* {
 -webkit-box-sizing: border-box;
 -moz-box-sizing: border-box;
 box-sizing: border-box;
 *behavior: url('/test/grid/script/boxsizing.htc');
}


/* Display block per gli elementi HTML5 su IE */
article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
nav,
section {
 display: block;
}


/* Immagini responsive */
img { 
 border:0; 
	vertical-align:middle; 
	-ms-interpolation-mode:bicubic;
}
 
img.scala { 
 max-width:100%; 
	height:auto; 
}


img.imgtext { 
 max-width:100%; 
	height:auto; 
	float: left;
	padding: 0 20px 15px 15px;
}


.table-responsive table td {width: 24%; word-break: break-all;}




/* Stili di base e tipografia */
html {
  font-size: 100%;
}


body {
 margin: 0;
 padding: 0;
font-family: ‘Times New Roman’, Times, serif;
 font-size: 16px;
 line-height: 20px;
 color:#000; 
 background-color: #fff;
 -webkit-text-size-adjust:100%;
}


p.text {
padding: 0 15px 0 15px;
text-align: justify;
}


p.foottext {
margin: 10px 10px 10px 10px;
padding: 20px 20px 20px 20px;
text-align: center;
}


a {
 color: #000;
 text-decoration: none;


}


a:hover {
 color: #fff;
 text-decoration: none;
 background:  url(../img/dark-bg1.png) repeat top;
 
}


h1, h2, h3, h4, h5, h6 { 
	font-family:"Georgia", "Times New Roman", serif; 
	color: #000;
} 


h1 { 
 font-size:42px; 
	line-height:46px; 
	margin-bottom:20px; 
}


h2 { 
 font-size:28px; 
	line-height:32px; 
	margin-bottom:12px; 
}


h3 { 
 font-size:21px; 
	line-height:25px; 
	margin-bottom:8px; 
}


/* Griglia */


.riga {
 width: 100%; 
 /* max-width: 960px; */
 max-width: none;
 min-width: 720px; 
 margin: 0 auto;
 background: url(../img/grey-light.jpg) repeat left top; 
}


[class*='colonna-'] {
 float: left; 
 min-height: 1px; 
 padding: 0 15px; 
 background: url(../img/grey-light.jpg) repeat left top;
}




.colonna-left {
margin: 10px 0 0 12px;
 width: 20%;
 height: 100%;
 background: url(../img/grey-light.jpg) repeat left top;
}


.colonna-right {
 margin: 10px 14px 10px 10px;
    float: right;
    width: 75%;
	 background: url(../img/grey-light.jpg) repeat right top;


}


.colonna-all {
float: left;
    width: 100%;
	
	text-indent: 10px;
	text-align: justify;
	/* margin: 0 15px 0 15px; */
		 background: url(../img/grey-light.jpg) repeat top;


}


.colonna-1-4 {
width: 25%;
}


.colonna-1-3 {
 width: 33.33%;
}


.colonna-1-2 {
 width: 50%;
}


.colonna-1 {
 width: 100%;
}


[class*="colonna-"] + [class*="colonna-"]:last-child {float: right;}


/* Micro-clearfix */
.riga:before,
.riga:after {
 content: " "; 
 display: table;
}


.riga:after {
 clear: both;
}


.riga {
 *zoom: 1;
}


/* Stili  per le varie sezioni */
header {
 height: 170px;
 border-bottom: 1px solid #333;
 position: relative;
 	 background: url(../img/header.jpg) repeat top;


}


header #logo {
 float: left;
 margin: 20px 0 20px 0;
}




footer {
 border-top: 1px solid #333;
 border-bottom: 1px solid #333;
	 background: url(../img/dark-bg1.png) repeat top;
	 color: #fff;


}


a.footer {
color: #fff;
}


nav {
 float: left;
 margin: 10px 0 0 0;
 padding: 20px 20px 20px 20px;
color:  #fff;
}


nav ul, nav li {
 margin: 0;
 padding: 0;
}


nav ul li {
 list-style: none;
 float: left;
 margin-right: 5px;
}


nav ul li a {
 display: block;
 padding: 5px;
}


a.attiva-nav {
 display: none;
}


@media only screen and (max-width:600px) {
.riga { 
 width: auto;
 min-width: 0; 
 margin-left: 0; 
 margin-right: 0;
}
 
[class*="colonna-"] { 
 width: auto !important; 
 float: none;  
}


[class*="colonna"] + [class*="colonna"]:last-child {float: none;}
 
[class*="colonna-"]:before, [class*="colonna-"]:after {
 content:""; 
 display:table;
}


[class*="colonna-"]:after {clear: both;}


a.attiva-nav {
 display: block;
	float: right;
	color: #2cb7ea;
	background: #4e4e4e;
	text-decoration: none;
	margin: 20px 0 0 0;
	padding: 0 10px;
	font-size: 14px;
	font-weight: bold;
	line-height: 22px;
	height: 22px;
}


nav {
 clear: both;
 position: absolute;
	top: 70px;
	width: 100%;
	z-index: 10000;
	background: #fff;
	padding: 0;
	display: none;
}


nav ul {
 margin: 0;
 padding: 0;
}
nav ul li {
 list-style: none;
 float: none;
}


nav ul li a { 
 padding: 5px;
 display: block;
 border-bottom: 1px solid #333; 
 background-color: #e0e0e0;
}
}
Grazie mille!