/usr/share/source-highlight/xhtml_common.outlang is in libsource-highlight-common 3.1.7-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | include "html_common.outlang"
bold "<span style=\"font-weight: bold\">$text</span>"
italics "<span style=\"font-style: italic\">$text</span>"
underline "<span style=\"text-decoration: underline\">$text</span>"
color "<span style=\"color: $style\">$text</span>"
bgcolor "<span style=\"background-color: $style\">$text</span>"
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