DIV not shrinking when resizing - html

Question

So I have a site with a simple two DIV panes: a content area left and a fixed 300px menu right, both of them going to a max of 1200px. I want users to be able to resize the window and have the LEFT pane shrink with the right menu staying fixed. But right now I can't find any way to do this, everything looks good at max size, but the left pane doesn't shrink if I resize the window, instead the right menu just wraps to the bottom of the screen. This would be easy with a left menu but the menu is on the right. Here is what I have so far:
#main
{
max-width: 1200px;
margin-right: auto;
margin-left: auto;
margin-top: 0;
display: block;
padding: 0;
}
#left
{
max-width: 890px;
float: left;
padding-right: 10px;
}
#right
{
width: 290px;
top: 0;
float: right;
padding-right: 10px;
padding-left: 10px;
}

Answer 1

you can use CSS calc() to adjust the width of the left container.
OPTION 1 FIDDLE
HTML
<div id="main">
<div id="left"></div>
<div id="right"></div>
</div>
CSS
#main{
width: 100%; //set to 100% since you're capping it at 1200 anyways
max-width: 1200px;
/* margin-right: auto;
margin-left: auto;
margin-top: 0; */ condense these to the following:
margin: 0 auto;
/*display: block;*/ already a block element so not necessary
padding: 0;
overflow: hidden; //add to correct floating elements
}
#left{
background: red; //just for my test
height: 100px; //just for my test
width: calc(100% - 300px); //readjusts based on screen size
float: left;
padding-right: 10px;
-moz-box-sizing: border-box; //if you use padding add these lines to fix issue of padding adding to width
-webkit-box-sizing: border-box;
box-sizing: border-box;
}
#right{
background: black; //just for my test
height: 100px; //just for my test
width: 300px;
/*top: 0;*/ //dont need, not doing anything
float: right;
/*padding-right: 10px;
padding-left: 10px;*/ //can condense to following:
padding: 0 10px;
-moz-box-sizing: border-box; //see padding explanation above
-webkit-box-sizing: border-box;
box-sizing: border-box;
}
OR
If you are worried about older browsers you can do this with display: table and display: table-cell like so:
OPTION 2 FIDDLE
CSS
#main{
width: 100%;
max-width: 1200px;
margin: 0 auto;
display: block;
padding: 0;
display: table; //add
table-layout: fixed; //add
}
#left{
display: table-cell; //use instead of float
background: red;
height: 100px;
padding-right: 10px;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
}
#right{
display: table-cell; //use instead of float
background: black;
height: 100px;
width: 300px;
padding: 0 10px;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
}

Answer 2

Instead of floating the divs, try display: inline-block on the left and right panes, or CSS3 flexbox (depends on how far back you support legacy browsers).
Flexbox example: http://jsfiddle.net/571k3gx2/