As we all know, IE7/8 does not really support XHTML. My question is: if I begin my XHTML document with "<?xml version="1.0" encoding="UTF-8"?>", will IE go into quirks mode despite the MIME type being "text/html"? Many thanks.

PS Not mentioning the deceased IE<=6.

Yes it will.

HOWEVER unless you are changing the version from 1.0 and the encoding from UTF-8, the xml declaration can be left OFF, as version 1.0 and UTF-8 encoding are the defaults without the declaration.