Force variable height DIV below variable height DIV - html

Question

I have a 2 horizontal part menu bar, each with a different background color.
The top bar has a menu system that will wrap if the screen is too small.
The second bar has an image and search bar.
The top bar needs to auto-size based on how many rows of options exist.
Currently the second bar's contents show up (image/search), however the background and DIV element are under the main bar.
<div id="header_nav_buttons" style="position:absolute">
<ul>
<li><asp:HyperLink ID="link1" runat="server"/></li>
<li><asp:HyperLink ID="link2" runat="server"/></li>
</ul>
</div>
<div id="sub_header">
<div id="logo">
<asp:Image runat="server" ID="img_Logo" ImageAlign="Left" Width="173px" />
</div>
<div id="search_bar">
<input name="search_input" runat="server" type="text" id="search_input" style="margin-right: 20px;" maxlength="50" />
<asp:Button ID="btn_Search" runat="server" Text="Search" CssClass="form_save_button" OnClick="search_go_Click" />
<br />
<asp:Label ID="lbl_Search_Box_Subtext" runat="server" />
</div>
</div>
CSS:
#header_nav_buttons
{
font-family: Verdana, Arial, Helvetica, sans-serif;
color: white;
float: left;
width: 100%;
background-color:#6690BF;
height: auto;
font-size: 1.5em;
line-height: 30px;
bottom: auto;
}
#header_nav_buttons ul
{
display: inline;
list-style: none;
margin: 0px;
}
#header_nav_buttons li
{
display: inline-flex;
padding-right: 13px;
}
#sub_header
{
height: auto;
width: 100%;
background-color: #e5ecf4;
}
#logo
{
width:173px;
padding-top: 15px;
}
#search_bar
{
right:20px;
margin-top: 25px;
float: right;
}
To add to the confusion, in #search_bar, if I remove float:right then some of the background shows up (while still starting too high, under the main header bar color).

Answer 1

I was able to solve this through some random attempts.
I added float:left to the #sub_header CSS section, and it solved the issue,