quick comments on Matrix

https://dvcs.w3.org/hg/FXTF/raw-file/default/matrix/index.html

In the array/sequence Matrix constructors, what happens if the length of
the array/sequence is not 6 or 16?

In the string constructor, what happens if the string is not valid?

Why don't you add the default parameters for Matrix methods such as scaleBy
to the WebIDL directly?

In rotateFromVector(By), what happens if the x or y coordinate is zero?
Actually it's not clear to me why these can't be zero.

I assume toFloat32/64Array always return a 16-element array but this should
be stated.

What's the rationale for using double for matrix elements? Are
implementations allowed to use 32-bit precision internally?

Rob
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