Aligning Web Forms - html

Question

I have two forms that I want to align. I was able to figure out how to align the height, but I can't get the width to work the way I want.
In my situation, I want the left box to fill in the remaining space. Using 'auto' just fills in the space to fit the contents. So instead of an expanding form, there is a gap between the two forms.
The reason I require this is I also have PHP around this form. There is a flag that dictates whether or not the second form shows up. So if there is only one form, I want it to expand the entire space. If there is two forms, I want them to share the space.
The way I thought of doing this would be to set the right form to a specific width and have the left form mold to whether or not anything else exists. I just can't figure out how to get the left form to expand itself without specifying an exact width.
I've included the HTML and CSS below as well as a JSFiddle reference.
HTML
<div class="section">
<div class="container">
<form>
<fieldset class="left">
<legend>PC Management</legend>
<div>
<input type='submit' value='Backup' />
<input type='submit' value='Backup' />
<input type='submit' value='Backup' />
</div>
</fieldset>
</form>
<form>
<fieldset class="right">
<legend>PC Management</legend>
<div>
</div>
</fieldset>
</form>
</div>
</div>
CSS
.section {
margin: 0 auto;
text-align: center
}
.container {
height: 100px;
width: 500px;
}
.container form, .container fieldset, .container input {
height: 100%;
display: inline;
}
.left {
width: auto;
float: left;
}
.right {
width: 40%;
float: right;
}

Answer 1

display: flex will get you the dynamic resizing that you want without needing any JavaScript:
.section {
margin: 0 auto;
text-align: center
}
.container {
height: 100px;
width: 500px;
display: flex;
}
.container form {
flex: 1 1 auto;
}
.container form, .container fieldset, .container input {
height: 100%;
}
<div class="section">
<div class="container">
<form class="left">
<fieldset>
<legend>PC Management</legend>
<div>
<input type='submit' value='Backup' />
<input type='submit' value='Backup' />
<input type='submit' value='Backup' />
</div>
</fieldset>
</form>
<form class="right">
<fieldset>
<legend>PC Management</legend>
<div>
</div>
</fieldset>
</form>
</div>
</div>
http://jsfiddle.net/eeLfx9d6/1/