pixel value for a border-width set to for example thin is incorrect.

the following code does the trick.

var toPixels = function ( element , value )
    {
        if ( /^(\\d+)|(small|medium|large|thin|thick)$/ . test ( value ) )
        {
            var tmp = document.createElement ( 'span' );
            tmp.style.position = 'absolute';
            tmp.style.visibility = 'hidden';
            tmp.style.borderStyle = 'solid';
            tmp.style.borderRightWidth = '0px';
            tmp.style.borderLeftWidth = value;
            element.parentNode.appendChild ( tmp );
            value = tmp.offsetWidth + 'px';
            element.parentNode.removeChild ( tmp );
        }
       
        return value;
    };