Compabilité entre ff, ie et opera

Bonjour,

J'aimerais que mon site : http://guillaumerase.be/spip/ soit compatible sous ie et ff parce que sous opera il fonctionne comme je veux pour le moment.
voici le css:

@charset "utf-8";
/* CSS Document */

html, body {
margin: 0px;
padding: 0px;
background-color: #0080e6;
}

/* CSS Structure */

#Entete {
padding: 0px;
height: 27px;
margin-left: auto;
margin-right: auto;
margin-top: 20px;
width: 1200px;
margin-bottom: 20px;
}

/* menuder.css */
#Entete .menuder-container {}
#Entete .menuder,
#Entete .menuder ul {
float: left;
width: 1200px;
margin: 0;
padding: 0;
list-style: none;
text-align: left;
font-weight: bold;
background-color: #F00;
}

#Entete .menuder a {
display: block;
width: 10em;
w\idth: 6em;
padding: .25em 2em;
text-decoration: none;
}

#Entete .menuder a.daddy {
background: url(img/menuder.gif) center right no-repeat;
}

#Entete .menuder li {
float: left;
width: 10em;
padding: 0;
}

#Entete .menuder li ul {
position: absolute;
left: -999em;
height: auto;
width: 14.4em;
w\idth: 13.9em;
margin: 0;
border-width: .25em;
font-weight: normal;
z-index:100;
}

#Entete .menuder li li {
width: 13em;
padding-right: 1em;
}

#Entete .menuder li ul a {
width: 13em;
w\idth: 9em;
}

#Entete .menuder li ul ul {
margin: -1.75em 0 0 14em;
}

#Entete .menuder li:hover ul ul,
#Entete .menuder li:hover ul ul ul,
#Entete .menuder li:hover ul ul ul ul,
#Entete .menuder li.hover ul ul,
#Entete .menuder li.hover ul ul ul,

#Entete .menuder li.hover ul ul ul ul {
left: -999em;
}

#Entete .menuder li:hover ul,
#Entete .menuder li li:hover ul,
#Entete .menuder li li li:hover ul,

#Entete .menuder li li li li:hover ul {
left: auto;
}

#Entete .menuder li.hover ul,
#Entete .menuder li li.hover ul,
#Entete .menuder li li li.hover ul,

#Entete .menuder li li li li.hover ul {
left: auto;
}

#Entete .menuder li:hover,

#Entete .menuder li.hover {
background-color: #1c8aca;
}
/* end */

#Contenu {
padding: 0px;
margin-left: auto;
margin-right: auto;
margin-top: 5px;
height: 486px;
width: 1200px;
background-color: #FFF;
}

#Contenu #CadreBanniere {
height: 486px;
width: 336px;
float: left;
z-index: 200;
background-image: url(../imgs/rase.jpg);
background-repeat: no-repeat;
background-position: left;
overflow: auto;
border-right-width: 5px;
border-right-style: solid;
border-right-color: #03F;
}

#Contenu #CadreTexte {
height: 476px;
width: 849px;
float: left;
margin-top: 10px;
margin-left: 10px;
text-align: left;
background-image: url(../imgs/fond_pave.jpg);
background-repeat: no-repeat;
}

#Contenu #CadreTexte #News {
height: 289px;
width: 849px;
float: left;
text-align: left;
overflow-y: scroll;
}

#Pied {
padding: 0px;
width: 1200px;
background-color:#FFF;
margin-top: 20px;
margin-left: auto;
margin-right: auto;
text-align: center;
}