wordpress page – positioning elements

My wordpress page template is one without columns(left or right sidebar etc.).
When I try to insert two div elements to my page through wordpress rich text editor and style with css it becomes weird:

<div id='first'>lalalalala</div>
<div id='second'>lalalalala</div>

CSS:

div#first{
width:600px;
border-right:1px solid black;
float:left;
}
div#second{
width:200px;
margin-left:10px;
border-left:1px solid black;
}

What is werid? div#second is not positioned next to the #first one, but instead underneath it.

Parent for both of these is more than 800px in width.