CSS menu dont work in IE - css

Question

what have I done wrong that in IE the menu doesnt work as in other browsers it does work.
In IE the menu buttons are vertical instead of horizontal.
Link to a test subdomain : http://test.beterlopenwinkel.nl/leiderdorp.php
And I cant get rid of that list style in IE neither.
CSS:
#navigation{
width:200px;
background-image: -moz-linear-gradient(left, orange, #f3980b); /* FF3.6 */
background-color:orange;
border-radius:8px;
font-size:16px;
font-weight:bold;
text-align:center;
width:1000px;
height:30px;
}
#navigation ul{
margin:0px;
padding:0px;
}
#navigation ul li{
display:inline;
list-style-type:none ;
padding:5px 10px 5px 10px; //dropmenupadding
margin-left:15px;
position:relative;
}
#navigation li a{
color:#fff;
text-decoration:none;
padding:5px 10px 5px 10px;
display:inline;
}
#navigation li a:hover{
text-decoration:none;
//background-color:orange;
//border-radius:15px;
//border:solid 2px #333;
color:#333;
}
#navigation li ul{ //dropmenu
margin:0px;
padding:0px;
display:none;
position:absolute;
left:0px;
top:35px;
background-color:orange;
border-radius:15px;
box-shadow:0px 0px 5px #000;
}
#navigation li:hover ul{
display:block;
width:160px;
}
#navigation li li{
list-style:none;
display:list-item;
}
#navigation li li a{
color:#fff;
text-decoration:none;
padding:10px;
}
#navigation li li a:hover{
text-decoration:none;
}

Answer 1

For starters, you might want to make sure your HTML is valid. Check the W3 Validator here:
http://validator.w3.org/check?uri=http%3A%2F%2Ftest.beterlopenwinkel.nl%2Fleiderdorp.php&charset=%28detect+automatically%29&doctype=Inline&group=0
Invalid markup is the #1 reason why CSS behaves in unexpected manners in different browsers (not just IE)

Answer 2

First, you need to fix the HTML:
<ul>
<li>Solidus</li>
<li>Finn Comfort</li>
<li>Think!</li>
<li>Footnotes</li>
<li>MBT</li>
<li>Vabeene</li>
<li>Meindl</li>
<li>Stadler</li>
<li>Fit Flop</li>
<li>Durea</li>
<li>Van Lier</li>
<li>Schneider</li>
</li> **THE LI SHOULD BE CLOSED AFTER THE UL, LIKE YOU DID FOR OTHER SUBMENUs**
</ul>