Hide / Reveal Divs not side by side - html

Question

I am trying to do a hide / reveal using javascript and css and my divs are stacking rather than lining up side by side. i have set a width and floats... i cant figure out what's going on. any help is greatly appreciated.
#container {
width: 760px;
margin: 20px auto;
padding: 30px;
background-color: #000;
border-width: 0px;
color: #fff;
}
#1a {
width: 300px;
float: left;
margin:10px
background-color: #000;
}
#1b {
width: 400px;
margin: 10px;
background-color: #000;
}
and the html:
<div id="container">
<div id="1b" class="hidden">
Module Details:
My First Page
</div>
<div id="1a">
01
</div>
i've been messing with it a lot, and now the second div is in the middle of the first... so here is a link if that's helpful too:
http://www.amandasmithsf.com/m14_SMITH_demo/test.html

Answer 1

Update
Inspecting your CSS in Firebug, I noticed that it wasn't being applied, then I saw why -- HTML IDs should not start with a number (or at least, not if you want them to work the CSS # selector; it turns out that in HTML5 they decided to start allowing IDs beginning with numbers, but you'll have to use a different strategy to select them with CSS: http://benfrain.com/when-and-where-you-can-use-numbers-in-id-and-class-names/).
Starting the IDs with letters instead of numbers made it work:
<style>
.hidden {display:none}
#container {
width: 760px;
margin: 20px auto;
padding: 30px;
background-color: #000;
border-width: 0px;
color: #fff;
}
#b1 {
float: left;
width: 300px;
margin:10px;
background-color: #000;
}
#a1 {
float:left;
width: 10px;
margin: 10px;
background-color: #000;
}
</style>
<script>
function unhide(id) {
document.getElementById(id).style.display = 'block';
}
</script>
<div id="container">
<div id="b1" class="hidden">
Module Details:
My First Page
</div>
<div id="a1">
01
</div>
</div>
Original Answer
I think you just need to add float: left; to #1b.
Or, if for some reason you really only wanted to assign float: left; to one of them, it would need to be #1b - the floated element needs to come before the non-floated element next to which you want it to display.