Anyone know why this :after CSS isn't working in IE9?

Question

This submit button should be rounded on the left side, and pointed on the right side. It's working in non-ie browsers, but in IE9, it is not working.

If I look at the styles in the developer tools, the .flat-button:after rule has everything crossed out, as if it is superseded by something. What?

<button type="submit" class="flat-button">Submit</button>
<style>
.flat-button {
    float: left;
    position: relative;
    border-top-left-radius: 5px;
    -moz-border-top-left-radius: 5px;
    -webkit-border-top-left-radius: 5px;
    border-bottom-left-radius: 5px;
    -moz-border-bottom-left-radius: 5px;
    -webkit-border-bottom-left-radius: 5px;
    border: none;
    padding: 0 12px;
    margin: 0 3px 3px 0;
    outline: none;
    cursor: pointer;
    color: white;
    font-family:'Trebuchet MS', Arial, helvetica, Sans-Serif;
    font-size: 14px;
    font-weight: bold;
    font-style: italic;
    text-decoration: none;
    border-collapse: separate;
    height: 26px;
    line-height: 26px;
    background: #5191cd;
}
.flat-button:hover {
    background: #1c3f95;
}
.flat-button:after {
    position: absolute;
    content: ' ';
    height: 0;
    width: 0;
    left: 100%;
    border: 13px solid transparent;
    border-left-color: #5191cd;
}
.flat-button:hover:after {
    border-left-color: #1c3f95;
}
</style>

Answer 1

After playing around for a while, I found a simple fix for IE9.

http://jsfiddle.net/thirtydot/BWC9q/10/

All you have to is add overflow: visible to .flat-button.