Questions about indentshift

I was looking at some of the linebreak tests in the MathML3 test suite, and some of them confuse me.  For example,

	http://www.w3.org/Math/testsuite/build/main/Topics/LineBreak/linebreaking011-indentamount-full.xhtml

starts with 

	<math xmlns="http://www.w3.org/1998/Math/MathML" indentshift="3em" display="block">

and has some <mi> and <mo> elements then 

	<mspace linebreak="newline"/>

According to 

	http://www.w3.org/TR/MathML3/chapter3.html#presm.lbindent.attrs

the default for indentshiftfirst is indentshift, and since indentshift is set to 3em, it seems to me that the first and second lines both should be indented to 3em.  But the image of the correct rendering shows the first line not indented, while the second line is indented 3em.  What am I missing, here?

A second confusion for me is in 

	http://www.w3.org/Math/testsuite/build/main/Topics/LineBreak/linebreaking010-indentshift-full.xhtml

where indenshiftlast is set to -20pt.  The image shows the last line right-aligned (with what appears to be about 20pt of space at the right, so it seems that a negative indentshift is supposed to mean indenting from the opposite side from what indentalign specifies?  There is no mention of this in the spec that I can find.  Can someone clarify the intended meaning of negative indents?

Finally, the tests seem to assume a default indentalign of left, but the spec indicates that the default is up to the renderer.  MathJax uses centering by default, so the tests "fail".  it might be good if the tests set indent align explicitly.

Davide