This is stated on this page http://www.veiled-chameleon.com/xhtml11.html
XHTML 1.1 allows for the easy creation of different display formats, such as a page format for printing, for wireless devices and PDAs, and television based browsers, simply by writing a new CSS (cascading style sheet) for the new document.
Update:
And this is stated on http://www.w3schools.com/w3c/w3c_xhtml.asp
XHTML 1.1 (Modular XHTML)
Small devices cannot support all XHTML functions, therefore XHTML 1.1 divides the specification into modules with limited functionality.
Small browsers can reduce their complexity by supporting only selected modules
Can anyone explain me what authors are saying in this statement and is this can't be done by XHTML 1.0 stric or HTML 4.01 Strict?