Technically CSS (Cascading Style Sheers) is part of HTML but it is quite different in many ways. It was added much later to provide the elaborate formatting that commercial web publishers want. None of this was necessary for the original HTML for academic publishing envisioned by the worldwide web developers.
The Willard textbook is unusual as it introduces CSS from the beginning. Most textbooks and courses add CSS after covering all the basic HTML functions. Yes, this approach makes getting started more difficult, but it is important to consider formatting for an entire page and/or site from the outset.