Model answers for almost all of the exercises can be downloaded as a zip file here (save the target of that link as file answers.zip and then unzip it).
The Mozilla organisation has a very useful JavaScript reference site.
Another reference for JavaScript objects, functions and variables is provided by w3schools JavaScript. I find this less complete and useful than the Mozilla site.
The full JavaScript standard (Ecma-262) may be downloaded as a PDF file from Ecma's site here. That covers the core language but not the objects comprising the DOM, or their functions.
The DOM is a World Wide Web Consortium standard and all the formal documentation can be found on the W3C site here.
The complete standard for HTML5 can be found at WHATWG (The Web Hypertext Application Technology Working Group). Previous standards and that for CSS can be found at W3C (The World Wide Web Consortium).
More about MOTW (the Mark Of The Web), can be found on microsoft.com, here.
Another introduction to HTML for beginners can be found on the "Who is hosting this?" site.
If you already know some HTML but need a reference then HTML.com is useful. You can find more about CSS and JavaScript there too.
The World Wide Web Consortium (W3C) absorbed the WHATWG's recommendations into their HTML5 Standard. Work is continuing on minor revisions and WHATWG (The Web Hypertext Application Technology Working Group) is still in action.
It has been pointed out to me that the w3schools site mentioned above does not comply with the World Wide Web Consortium's Accessibility Guidelines and so may not be practical for visually impaired users.