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[xml] Adding default DOCTYPE when HTML document doesn't have any

Hi, I use libxml to do HTML processing using htmlParseDocument, than do
some simple transformations (like replacing URIs just to correct relative
patch etc.) and then save the document using xmlSaveDoc(). The output is
an HTML file that is passed to the web browser.

The problem is that in case that there is no DOCTYPE declaration in the
input document libxml2 adds a default one:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
        "http://www.w3.org/TR/REC-html40/loose.dtd";>

There is a difference in rendering of pages by web browsers that comes
from various quirks modes that are turned on or off based on the DOCTYPE
declaration. To illustrate the difference there is a test page where you
can see the same HTML/CSS code with various DOCTYPEs prepended:

http://dbaron.org/mozilla/tests/compat?doctype=
http://dbaron.org/mozilla/tests/compat?doctype=%3C!DOCTYPE+HTML+PUBLIC+%22-%2F%2FW3C%2F%2FDTD+HTML+4.01+Transitional%2F%2FEN%22+%22http%3A%2F%2Fwww.w3.org%2FTR%2Fhtml4%2Floose.dtd%22%3E
http://dbaron.org/mozilla/tests/compat?doctype=%3C!DOCTYPE+HTML+PUBLIC+%22-%2F%2FW3C%2F%2FDTD+HTML+4.01+Transitional%2F%2FEN%22%3E
http://dbaron.org/mozilla/tests/compat?doctype=%3C!DOCTYPE+HTML%3E

Although that in the cases I've seen the web page having no DOCTYPE is
rendered like with the DOCTYPE that is prepended by libxml2 I would be
happy if there was a way to not append the default DOCTYPE or to
know that the original document had no DOCTYPE at all. Is there a
way to do that?

-- 
Damian Pietras

http://www.linuxprogrammingblog.com
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