It is in "extensions\WikiSyntax.py"
At about line 52 you will see:
HtmlTagRE = re.compile(
        ur"</?[A-Za-z][A-Za-z0-9]*(?:/| [^\n>]*)?>",
        re.DOTALL | re.UNICODE | re.MULTILINE)
replace it by:
HtmlTagRE = re.compile(
        ur"</?[A-Za-z][A-Za-z0-9:]*(?:/| [^\n>]*)?>",
        re.DOTALL | re.UNICODE | re.MULTILINE)
Michael
Be aware that the content of the "extensions" directory is overwritten
each time a new version of WikidPad is installed. For your concrete
change this doesn't matter because I include the change in the
installation files for the next version.
If you want to make further changes, you should create a directory
"user_extensions" (as a sibling of "extensions" dir.) if not already
there. Copy "WikiSyntax.py" to "user_extensions" and change it there.
Michael