valign attribute in style classes

When we define a style class like the below in a CSS file, it is getting rendered in IE, but FF shows empty classes. vertical-align attribute w开发者_运维问答orks fine in both browsers. Why do we see this behaviour?

.text {
   valign: middle;
}

Because valign isn't valid CSS, while vertical-align is. And IE is known to ignore the rules. What more?