Check, Parse:
xmlwf file.xml      checks if well-formed.    in expat rpm

xmllint file.xml    parses.                   in libxml rpm
   --debug          annotated parse tree
   --noout
   --valid          access DTD to validate document

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PHP:
<?php header('Content-Type: text/xml'); ?>
<?php     //must echo the xml declaration
echo '<?xml version="1.0" encoding="ISO-8859-1" ?>'."\n";
?>

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MathML:

<!DOCTYPE math 
    PUBLIC "-//W3C//DTD MathML 2.0//EN"
           "http://www.w3.org/Math/DTD/mathml2/mathml2.dtd"
>
<math xmlns="http://www.w3.org/1998/Math/MathML">

Linux Firefox says must install various fonts.  but puts the variables
in correct positions


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SVG:
display file.svg
rsvg-view  file.svg            svg viewer
         -x xzoom -y yzoom      -w width -h height
rsvg infile.svg outfile.png    convert svg to png

http://plugindoc.mozdev.org/linux.html#SVG
XP Firefox: get version 6 Adobe SVG Viewer

<embed src="threeShapes.xml" width="800" height="800" type="image/svg-xml" />


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Browsers:
Linux Firefox shows parse errors of xml file, else displays text.
  DOM inspector has parse tree.
XP: IE/Firefox shows parse tree.

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Emacs:
added these to .emacs:
(add-hook 'sgml-mode-hook '(lambda() (font-lock-mode 1)))
(add-hook 'xml-mode-hook '(lambda() (font-lock-mode 1)))

Must load a .html before a .xml!!!! else neither xml nor html highlighted