A lot of things from HTML 4 can be written shorter in HTML 5
Defining the doctype and meta tag maens that the website doesn't have to guess what you want.
Javascript allows you to manipulate the DOM, react ot users or browser generated events, or make use of new APIs
Some new APIs include Canvases, audio and videos, local stroage, and more!
A design principle behind HTML5 is to allow pages to degrade gracefully (meaning if a browser doesn't support a new feature an alternative should be available.
With Javascript you can use geolocation, cache data locally
HeadFirst uses var rather than let. We use let whenever possible because var is scoped more specific
Overall, HTML 5 has many new features that will help progress web development. You don't need to worry about transfering to HTML5 too early since everyone else is doing it and HTML5 is built to allow older HTML to still work (just not as amazingly). Javascript allows programmers to make their pages much more interactive and allows them access to several APIs.