IE9 mode in IE10 does not match real IE9

This page requests IE9 mode using <meta http-equiv="X-UA-Compatible" content="IE=9">. document.documentMode says: .

There are three ways to rotate a square 45° below.

-ms-transform: rotate(45deg);

Supported by IE9 and the IE9 mode in IE10.

transform: rotate(45deg);

Not supported by the real IE9 but supported by the IE9 mode in IE10. Oops!

transform: rotateZ(45deg);

Supported by the IE10 mode. Consistently not supported by either the real IE9 or the IE9 mode in IE10.