HTML versus XML rendering of webpage

[Jesper Tverskov <jesper.tverskov@gmail.com>]

HTML versus XML rendering of webpage

I'm trying to test what it true and false about XML and HTML rendering
of webpages. The conventional wisdom is, that XML rendering is
marginally faster because the parser is smaller but that XML rendering
can give problems in the real world because it is not done
incrementally.

I have made a huge webpage, polyglot XHTML5, to test it. 100.000 lines
and 8-9 MB.

XML
http://www.xmlplease.com/xml/xhtmlhtml/

HTML
http://www.xmlplease.com/xml/xhtmlhtml/xhtmlhtml.html

The surprising result, at least to me, is, that all browsers, IE9,
Firefox, Opera, Crome and Safari use incremental rendering also for
XML. They all show the XML webpage almost instantly but it takes
20-40s to load everything. You can start scrolling right away.

Another less surprising fact: XML rendering for this type of document
is marginally faster than HTML rendering. For this monstrous document
the speed difference even matters in some of the browsers!

Questions. Have I overlooked something? Have the browsers fooled me?
What is the truth about XML and incremental rendering? What is the
difference in speed of XML and HTML rendering? Can anyone confirm my
test results?

I use exactly the same file for XML and HTML rendering. In the first
case a script tests if "application/xhtml+xml" is supported, and adds
"application/xhtml+xml" to the header. In the last case I just load
the webpage without using the script.

I do use "application/xhtml+xml". When I introduce a well-formedness
error in the last line, all browsers render the webpage except that
after 20-40s they show the XML error message instead!

Cheers
Jesper Tverskov
http://www.xmlplease.com