div not extending full width

Question

I have a basic page built on the standard twoColFixLtHdr template that comes with CS4. Have created a new div #welcome that should extend across the whole width of the mainContent div, and the math adds up and the view layout guides shows everything in place. When you use Design Live View or a browser, there is a 20px white space at the right end of the #welcome block. I have just included the basic definitions below as I am sure this is something simple that my limited experience is not seeing. Thanks in advance for any/all advice or suggestions.
.twoColFixLtHdr #container {
width: 760px;  /* using 20px less than a full 800px width allows for browser chrome and avoids a horizontal scroll bar */
background: #FFFFFF;
margin: 0 auto;
text-align: left; /* this overrides the text-align: center on the body element. */
.twoColFixLtHdr #sidebar1 {
float: left; /* since this element is floated, a width must be given */
width: 200px; /* the actual width of this div, in standards-compliant browsers, or standards mode in Internet Explorer will include the padding and border in addition to the width */
background-image: url(../Images/sidebar-graphic-bkgrnd.gif);
background-repeat: repeat-y;
/* the background color will be displayed for the length of the content in the column, but no further */
padding: 0;
position: relative;
top: 0px;
left: 0px;
*in the Spry Vertical Menubar.CSS I have added a 2px border around each menu item and sub item giving me a total of 204px width for the sidebar}
.twoColFixLtHdr #mainContent {
margin: 0 0 0 204px; /* the left margin on this div element creates the column down the left side of the page - no matter how much content the sidebar1 div contains, the column space will remain. You can remove this margin if you want the #mainContent div's text to fill the #sidebar1 space when the content in #sidebar1 ends. */
padding: 0; /* remember that padding is the space inside the div box and margin is the space outside the div box */
                position: relative;
                top: 0;
                left: 0;
.twoColFixLtHdr #container #mainContent #welcome {
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
color: #FFF;
width: 556px;
position: relative;
left: 0;
top: 0px;
background-image: url(../Images/welcome-graphic-bkgrnd.gif);
background-repeat: repeat-y;
float: left;

Answer 1

By the way I just used the same css as yours and it is working fine for me:
Following are the styles that I testerd for the said template:
body  {
     font: 100% Verdana, Arial, Helvetica, sans-serif;
     background: #666666;
     margin: 0; /* it's good practice to zero the margin and padding of the body element to account for differing browser defaults */
     padding: 0;
     text-align: center; /* this centers the container in IE 5* browsers. The text is then set to the left aligned default in the #container selector */
     color: #000000;
.twoColFixLtHdr #container {
width: 760px;  /* using 20px less than a full 800px width allows for browser chrome and avoids a horizontal scroll bar */
background: #FFFFFF;
margin: 0 auto;
text-align: left; /* this overrides the text-align: center on the body element. */
.twoColFixLtHdr #header {
     background: #DDDDDD;
     padding: 0 10px 0 20px;  /* this padding matches the left alignment of the elements in the divs that appear beneath it. If an image is used in the #header instead of text, you may want to remove the padding. */
.twoColFixLtHdr #header h1 {
     margin: 0; /* zeroing the margin of the last element in the #header div will avoid margin collapse - an unexplainable space between divs. If the div has a border around it, this is not necessary as that also avoids the margin collapse */
     padding: 10px 0; /* using padding instead of margin will allow you to keep the element away from the edges of the div */
.twoColFixLtHdr #sidebar1 {
float: left; /* since this element is floated, a width must be given */
width: 200px; /* the actual width of this div, in standards-compliant browsers, or standards mode in Internet Explorer will include the padding and border in addition to the width */
background-image: url(../Images/sidebar-graphic-bkgrnd.gif);
background-repeat: repeat-y;
/* the background color will be displayed for the length of the content in the column, but no further */
padding: 0;
position: relative;
top: 0px;
left: 0px;
/*in the Spry Vertical Menubar.CSS I have added a 2px border around each menu item and sub item giving me a total of 204px width for the sidebar*/}
.twoColFixLtHdr #mainContent {
margin: 0 0 0 204px; /* the left margin on this div element creates the column down the left side of the page - no matter how much content the sidebar1 div contains, the column space will remain. You can remove this margin if you want the #mainContent div's text to fill the #sidebar1 space when the content in #sidebar1 ends. */
padding: 0; /* remember that padding is the space inside the div box and margin is the space outside the div box */
position: relative;
top: 0;
left: 0;
.twoColFixLtHdr #container #mainContent #welcome {
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
color: #FFF;
width: 556px;
position: relative;
left: 0;
top: 0px;
background-image: url(../Images/welcome-graphic-bkgrnd.gif);
background-color:#330099;/*used this to see the div properly*/
background-repeat: repeat-y;
float: left;
Are your all the styles same as this?
Regards,
Vinay