This is the most comprehensive list of HTML tags I can find; it's searchable and easily links to short descriptions, so it should be the easiest one to use when you need to find a certain function.
This is a list of the most commonly used tags, so it may be more helpful than the HTML Reference when you need a refresher on the basic stuff.
This source gives a list of common CSS mistakes, so it'll be a useful tool to check back on to make sure everything is formatted and designed properly.
This is a searchable database of all CSS tags, similar to the prior HTML source by W3 Schools.
This blog post lists mistakes commonly seen in web design; while some are more about site strategy, there's a lot of helpful examples of what not to do in web design that could be used as references.
This post provides basic elements of color theory, and explains how they apply to web design.
This page lets you search for copyright free, reusable assets under public domain or Creative Commons.
This is another searchable database of royalty free assets like Openverse