To subscript text use sub tags
<?xml version = "1.0"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns = "http://www.w3.org/1999/xhtml"> <head> </head> <body> <p>Only <sub>one</sub> download per hour is permitted.</p> </body> </html>