Xara's variants option is pretty nifty but it's not a magical, one-button solution. Take a look in the Help file under Web Graphics and Websites/Responsive web design and read through the chapters. To make good use of the Styles options for the text in the variants, read the 'Text Styles' sections in the 'Text Handling' chapter.

You might first want to experiment with a dummy site to get the hang of it before you starting working on/building your own site.