This guide by Robert Mening is a great introduction to HTML5. It is formatted by dividing each piece of information into brief sections which make the learning curve flow in an easy way. Plenty of examples are provided to show exactly how all of it works.
The Ultimate Guide to HTML5 Is a more in depth, wordy look at HTML5 which discusses much more of the technical aspects in various ways. As opposed to Robert Mening's guide, this guide isn't so much for someone who wants to jump in and get going in a few hours at most.
Cssreference.io is a visual guide to CSS. In contains a plethora of CSS commands and allows you to simply click on them, obtain an explanation for that command, and see a visual representation of what that command is doing. It is a brilliate reference to bookmark for any CSS user.
CSS Beginner Tutorial on HTML Dog is more true to being a guide to CSS. It has several pages which explain what CSS is, its uses, and how it applies to HTML. Further on from this page you can go back and access HTML information or move forward to working with CSS.
Website Builder Expert's article on color schemes in website designs offers a straight forward, well detailed guide into picking a color design for your website. If offers statistics as to why this is an important issue, and how to get the look you're going for.
Thomas Channon's article discusses the theory behind color usage in websites. It is a more in depth discussin than the previous article, and Thomas immedietly notes that it is targeted at people looking to design their own website. Thomas uses several visual guides to demonstrate certain techniques and tactics to make the design of your website pop out in an effective and efficient way. This is no doubt a must read for anyone looking to get into web designing.
Pexels is a host website of images that are open to be used by anyone. Users can download royalty-free photos for free, as well as contribute photos to the website.
Pixabay is another free image hosting website. Like Pexels, it is easy to navigate and find what you're looking for. Pictures are easy to download and utilize immediately, for free.