The animal on the cover of HTML & XHTML: The Definitive Guide is a koala. The koala is an Australian marsupial, the only member of the Phascolarctidae family.
When they are born, koalas are tiny, weighing approximately 0.5 grams. A young koala stays in its mother's pouch for approximately seven months. Unlike most marsupials, the koala's pouch opens near the rear, not near the head. Koalas have a high mortality rate and face extinction in Australia due to epidemics in 1887–1889 and 1900–1903, and unrestrained hunting throughout the 20th century. They are a protected species. Populations are rebuilding, but at present, they survive only in eastern Australia.
The cover image is a 19th-century engraving from the Dover Pictorial Archive. The cover font is Adobe ITC Garamond. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont's TheSans Mono Condensed.