CSS Horizontal Menu: display: inline; won't work - css

Question

I'm on Chrome/Firefox. I've read several tutorials and questions here too on this topic.
I have a UL containing LI. I set the LIs to "display: inline;", but they won't. They're still vertical.
What am I doing wrong?
Thx.
Here's my css:
.menu{
width: 100%;
padding: 0px;
margin: 0px;
outline: 1px solid grey;
background-color: #f6f6f6;
font-size:100%;
}
.menu ul{
float: left;
list-style: none;
margin: 0px;
padding: 0px;
background-color: #f6f6f6;
}
.menu li ul{
display: none;
height: auto;
padding: 0px;
margin: 0;
background-color: #f6f6f6;
outline: 1px solid gray;
position: absolute;
z-index: 200;
left: 20px;
top: 30%;
}
.menu li:hover ul{
display: block;
}
.menu li:hover{
background-color: #005ea2;
}
.menu li{
display: inline;
padding: 0;
margin: 0;
border-bottom: 1px dotted grey;
}
.menu ul li:last-child{
border: none;
}
.menu a{
display: block;
color: #333333;
text-decoration: none;
margin: 0px;
margin-left: 5px;
}
.menu a:hover{
color: white;
background-color: #005ea2;
}
.menu .menu_header{
color: #333333;
}
.menu .menu_header a:hover{
color: white;
}

Answer 1

The <li> elements need to have float:left

Answer 2

first of all you might consider adding the relevant HTML to your answer, to help answerers understand your situation better.
I can spot several strange things that might be related to your problem.
First, the li ul selector are not very useful: it's more likely that you meant the opposite, ul li.
Second, I see that on hover you have something changing to display: block. This is a very
strange behavior to have on hover, are you sure?
Third, I see that you have a { display: block }. This does not play nicely if its container is display: inline so you might want to switch to display: inline-block for the container (which is what I would suggest for an horizontal menu anyway)