Fix issue #2141: adhere to the matrix-vector product convention (#2142)

* Fix issue #2141: matrix-vector product convention

* Revert "Fix issue #2141: matrix-vector product convention"

This reverts commit 1ecba3b.
Have a look at issue #2141 for comments.

* Clarification of the system handedness (#2141)

files/en-us/web/api/webgl_api/matrix_math_for_the_web/index.html

@@ -240,7 +240,7 @@ <h2 id="Rotation_matrix">Rotation matrix</h2>
 ];
 </pre>
 
-<p>It is possible to encode these type of steps into a matrix, and do it for each of the x, y, and z dimensions. Below is the representation of a rotation about the X axis:</p>
+<p>It is possible to encode these type of steps into a matrix, and do it for each of the x, y, and z dimensions. Below is the representation of a counterclockwise rotation about the Z axis in a left-handed coordinate system:</p>
 
 <pre class="brush: js">let sin = Math.sin;
 let cos = Math.cos;