IE9 css: first child

I have a css rule similar to this in my css file:

.Island_VerticalMenu:first-child { width: 100% }

However, when I view it in the IE9 developer tool, it looks like this:

:first-child.Island_VerticalMenu { width: 100% }

and does not work.

If I manually changed it to the first one in the developer tool, it will be applied correctly.

Any known workaround?

Update:

* > .Island_VerticalMenu:first-child { width: 100% }
* > table.Island_VerticalMenu:first-child { width: 100% }
table.Island_VerticalMenu:first-child { width: 100% }
table.Island_VerticalMenu > span  { width: 100% }
.Island_VerticalMenu:first-child { width: 100% }
.Island_VerticalMenu:first-child span { width: 100% }
.Island_VerticalMenu  span:first-of-type { width: 100% }
.Island_VerticalMenu > span { width: 100% }

None of these works

PS. I'm in compatibility mode