How to make on hover take 100% of page width up - html

Question

I have this navi bar... When I hover over an option I want it to take up 100% width so it spans across the entire screen. When I change the width amounts to 100% it just gets smaller, Not bigger.
HTML
<div id="cssmenu">
<ul>
<li class="has-sub"><span>Details</span>
<ul>
<li>
<?php include 'summary.php';?>
</li>
</ul>
</li>
<li class="has-sub"><span>.....</span>
</li>
</ul>
</div>
CSS
#import url(http://fonts.googleapis.com/css?family=Oxygen+Mono);
/* Starter CSS for Menu */
#cssmenu {
padding: 0;
margin: 0;
border: 0;
width: auto;
}
#cssmenu ul,
#cssmenu li {
list-style: none;
margin: 0;
padding: 0;
}
#cssmenu ul {
position: relative;
z-index: 597;
}
#cssmenu ul li {
float: left;
min-height: 1px;
vertical-align: middle;
}
#cssmenu ul li.hover,
#cssmenu ul li:hover {
position: relative;
z-index: 599;
cursor: default;
}
#cssmenu ul ul {
visibility: hidden;
position: absolute;
top: 100%;
left: 0;
z-index: 598;
width: 100%;
}
#cssmenu ul ul li {
float: none;
}
#cssmenu ul ul ul {
top: 0;
left: 190px;
width: 700px;
}
#cssmenu ul li:hover > ul {
visibility: visible;
}
#cssmenu ul ul {
bottom: 0;
left: 0;
}
#cssmenu ul ul {
margin-top: 0;
}
#cssmenu ul ul li {
font-weight: normal;
}
#cssmenu a {
display: block;
line-height: 1em;
text-decoration: none;
}
/* Custom CSS Styles */
#cssmenu {
background: #333333;
border-bottom: 4px solid #1b9bff;
font-family: 'Oxygen Mono', Tahoma, Arial, sans-serif;
font-size: 12px;
}
#cssmenu > ul {
*display: inline-block;
}
#cssmenu:after,
#cssmenu ul:after {
content: '';
display: block;
clear: both;
}
#cssmenu ul {
text-transform: uppercase;
}
#cssmenu ul ul {
border-top: 4px solid #1b9bff;
text-transform: none;
min-width: 1200px;
}
#cssmenu ul ul a {
background: #1b9bff;
color: #ffffff;
border: 1px solid #0082e7;
border-top: 0 none;
line-height: 150%;
padding: 16px 20px;
font-size: 12px;
}
#cssmenu ul ul ul {
border-top: 0 none;
}
#cssmenu ul ul li {
position: relative;
}
#cssmenu ul ul li:first-child > a {
border-top: 1px solid #0082e7;
}
#cssmenu ul ul li:hover > a {
background: #4eb1ff;
color: #ffffff;
}
#cssmenu ul ul li:last-child > a {
-moz-border-radius: 0 0 3px 3px;
-webkit-border-radius: 0 0 3px 3px;
border-radius: 0 0 3px 3px;
-moz-background-clip: padding;
-webkit-background-clip: padding-box;
background-clip: padding-box;
-moz-box-shadow: 0 1px 0 #1b9bff;
-webkit-box-shadow: 0 1px 0 #1b9bff;
box-shadow: 0 1px 0 #1b9bff;
}
#cssmenu ul ul li:last-child:hover > a {
-moz-border-radius: 0 0 0 3px;
-webkit-border-radius: 0 0 0 3px;
border-radius: 0 0 0 3px;
-moz-background-clip: padding;
-webkit-background-clip: padding-box;
background-clip: padding-box;
}
#cssmenu ul ul li.has-sub > a:after {
content: '+';
position: absolute;
top: 50%;
right: 15px;
margin-top: -8px;
}
#cssmenu ul li:hover > a,
#cssmenu ul li.active > a {
background: #1b9bff;
color: #ffffff;
}
#cssmenu ul li.has-sub > a:after {
content: '+';
margin-left: 5px;
}
#cssmenu ul li.last ul {
left: auto;
right: 0;
}
#cssmenu ul li.last ul ul {
left: auto;
right: 99.5%;
}
#cssmenu a {
background: #333333;
color: #CBCBCB;
padding: 0 20px;
}
#cssmenu > ul > li > a {
line-height: 48px;
font-size: 12px;
}
It needs to be 100% not like 1200px because I want it to fit correctly on different screens.

Answer 1

When you want to take a percent of the viewport, with CSS3 you can use VW (viewport width) and VH (viewport height)

Answer 2

to make the submenu have width as 100%, the #cssmenu should have position relative added to it. remove the position relative applied to the li.hover