This is where you start to do fancier things with CSS, such as borders. Also, this is when you learn how to work with pictures.
I hate to sound like a person who came from the 1980s-1990s, but I think the book should have gone further with the message on how you shouldn't assume the whole world has access to 1-100 gigabit Ethernet speeds. Heck, some areas of rural US can't even get anything close to 1 gigabit internet due to the ISP oligopoly that has severely imbalanced and is ruining the state of internet access. If you don't believe me, then watch the internet exclusive episode of Patriot Act with Hasan Minhaj on internet access in the US. And then take the next step of surmising/inferring to deduce that the internet infrastructure in the rest of the world might not even be as good as in a metropolitan area around a major US city.
Making images accessible is also a very good subtopic to pay attention to.