Input with display:block is not a block, why not?
Solution 1:
Check out what I came up with, a solution using the relatively unknown box-sizing:border-box
style from CSS 3. This allows a 'true' 100% width on any element regardless of that elements' padding and/or borders.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<title>Cross-browser CSS box-sizing:border-box</title>
<style type="text/css">
form {display:block; margin:0; padding:0; width:50%; border:1px solid green; overflow:visible}
div, input {display:block; border:1px solid red; padding:5px; width:100%; font:normal 12px Arial}
/* The voodoo starts here */
.bb {
box-sizing: border-box; /* CSS 3 rec */
-moz-box-sizing: border-box; /* Firefox 2 */
-ms-box-sizing: border-box; /* Internet Explorer 8 */
-webkit-box-sizing: border-box; /* Safari 3 */
-khtml-box-sizing: border-box; /* Konqueror */
}
</style>
<!-- The voodoo gets scary. Force Internet Explorer 6 and Internet Explorer 7 to support Internet Explorer 5's box model -->
<!--[if lt IE 8]><style>.bb {behavior:url("boxsizing.htc");}</style><![endif]-->
</head>
<body>
<form name="foo" action="#">
<div class="bb">div</div>
<input class="bb" size="20" name="bar" value="field">
</form>
</body>
</html>
This solution supports Internet Explorer 6 and Internet Explorer 7 via a behaviour written by Erik Arvidsson with some tweaks from Dean Edwards to support percentage and other non-pixel widths.
Working example
Behaviour (boxsizing.htc)
Solution 2:
The reason this happens is that a text input's size is determined by its size attribute. add size="50" inside the <input> tag. Then change it to size="100" -- see what I mean?
I suspect there's a better solution, but the only one that comes to mind is something I found on the "Hidden features of HTML" question on SO: Use a content-editable div, instead of an input. Passing the user input to the enclosing form (if that's what you need to) might be a little tricky.
Hidden features of HTML
Solution 3:
Your best bet is to wrap the input in a div with your border, margins, etc., and have the input inside with width 100% and no border, no margins, etc.
For example,
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>width:auto</title>
<style>
div {
border: 1px solid red;
padding: 5px;
}
input, form {
display: block;
width: 100%;
}
</style>
</head>
<body>
<form>
<div><input type="text" name="foo" /></div>
</form>
</body>
</html>