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Mozilla Bug Report For Negative Width in Mspace Command
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Claim
It appears to me that negative values for number are valid in the MathML command
     <mspace width="numberem"/>
commands. In addition, it appears to me that Mozilla version 1.2b for MS Windows is not displaying negative spaces correctly.

Reason for Negative Spaces
Here is the reason that it appears so to me. In Section 3.2.7.2 of
      http://www.w3.org/TR/MathML2/chapter3.html#presm_mspace
the value of the attribute width is specified as a number followed by an h-unit. In Section 2.4.4.1 of
      http://www.w3.org/TR/MathML2/chapter2.html#fund_attval
a number is defined as " decimal integer or rational number (a string of digits with one decimal point), optionally starting with '-' ".

Examples of Incorrect Display
A positive space display is demonstrated by the following input 
 
	<mi>x</mi><mi>y</mi><mspace width="1em"/><mi>z</mi>
which generates the output
      xyz
The negative spaces do not display correctly as demonstrated by the following input 
 
	<mi>x</mi><mi>y</mi><mspace width="-1em"/><mi>z</mi>
which generates the output
      xyz
Finally, note that a useful negative space example is included in the file
      http://www.w3.org/Math/testsuite/testsuite/Presentation/TokenElements/mspace/mspace2.xml
Input File: latex/NegativeSpace.omh