Ext.util.CSS.getRule doesn't appear to work in IE8 but it works in chrome.

My CSS
Code:
.x_3FieldsPerLine input, .x_3FieldsPerLine select {
    width: 227px;
}
My ExtJS
Code:
var classCssRule = Ext.util.CSS.getRule(".x_3FieldsPerLine input, .x_3FieldsPerLine select");	
var cssWidth = classCssRule.style.width;
classCssRule appears to be undefined in IE8 although in chrome it is object and cssWidth is 227px