Editing XSLT would be easier if HTML tags didn't "disturb" the overall view.
Most XML editors don't provide this feature. The only exception I know is an excellent plugin for jEdit, where HTML tags are all one colour (Navy blue) and XSLT syntax uses different shades of green for prefixes, elements, attribute names, light blue for attribute values and violet for functions. Very pleasant to work with.
FO code could also be different from XSLT, but even jEdit doesn't provide this feature.
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