Basically, a word is either an atom or quoted string. There's an ongoing, and fascinating debate raging in the XML community right now about the supposed efficiencies associated with binary versus textual data formats, based on the presumption that binary is always faster than text. Developers and markup jockeys can read the markup directly to figure out what's going on. This greatly lowers the probability that parser, interpreter, or runtime handling errors can systematically mangle such markup. Numerous XML developers report anecdotal and more formal evidence that in many cases, parsing and building the DOM for an XML document occurs faster than internalizing some binary representation of the same data.
Software is very good at processing plain text, and that's all good markup contains.
Deriving efficient, compact, and useful binary encodings is much more difficult than designing efficient, compact, and useful markup languages. Effective design of binary formats is nowhere near as well documented or understood as effective design of markup languages, either. Because they're anything but transparent binary formats are also inherently closed forms of notation. All too often, they're proprietary to boot. Properly constructed XML markup by contrast is open, transparent, readable, and seldom proprietary.