2.2.16 [W3C-XSLT] Section 16.2, HTML Output Method
C0021:
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The html output method may output a character using a character entity reference, if one is defined for it in the version of HTML that the output method is using.
MSXML3 and MSXML6
The html output method writes a character using a character entity reference, if one is defined for it in the version of HTML that the output method is using.