For the course, it is important to introduce all the new features of HTML and HTML5 as this is where web development is heading.
However, as you have discovered, this is really challenging. Everything is changing all the time. Some people and sites are using the latest and greatest new features. Other people haven’t changed or upgraded anything since they bought their computer years ago.
As a web developer, you have to understand the needs of your audience. In Willard p.207, there is information about providing fallback options. This is an example of how you, the web site developer can accommodate the needs of a broader audience. Some developers will do this. Others don’t bother. It is up to you.