The IE box model (known as the traditional box model) includes padding and an element’s width / height border.
In the IE box model, a field with a width of 100 pixels, with 2px padding on each side, a border of 3px and an edge of 7px on each side will have a visible width of 114px.
The W3C box model (which is the standard window model) excludes indentation and bordering from the width / height of the element.
In the W3C box model, a box with a width of 100 pixels, with a 2px gasket on each side, a 3px border and a 7px edge on each side will have a visible width of 124px.
Side models http://www.456bereastreet.com/i/box-model.gif
For IE to use the W3C window model (which any other browser uses), your page must be displayed in strict mode. By default, IE is displayed in Quirks mode.
To run Strict mode in IE, you must specify doctype. You can use any of the following doctrines:
HTML4 Strict:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd" >
XHTML 1.0 Strictly:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Your doctype should be the first that appears on your page. This is before the <html> on a separate line.
Additional information about Quirks / Strict mode:
CSS - Quirks mode and strict mode