Sitecore® XP 8 Website Development for .NET Developers eLearning Course resource zip Student Resources Folder (1).zip which you have to download doesn't include the sample images and js directories shown the videos.
So when you get further in the courses the instructions tell you to preview and validate in Experience Editor but you can't because nothing renders correctly. It doesn't render because pet-detail.html uses the references in the sample images and js folders. The only thing that renders is the footer.
Here's an example in Topic 3.4 Lab - Create two Sublayouts.pdf:
<img src="sample images/rhodesian.png" />
There are missing files in the Student Resources Folder. However, these were not included since you will be rewriting the original html files into Sitecore files in the labs. You will be using different images and the images will be pulled from the Sitecore database and not from hard drive files. In short, the Student Resource Folder does not contain all the original files that the html files reference, but that should not affect your progress through the labs.