What this is
These pages will show you how to construct web pages using a text editor and plain HTML5 code. (HTML stands for HyperText Markup Language.) While WYSIWYG editors are available to remove much of the tedium of web page construction, it is useful to understand the HTML product being created. This will allow you to fix poor constructions from editors, and appreciate clever constructions on other people's web pages.
I recommend the book Introducing HTML5 by Bruce Lawson and Remy Sharp. The book is compact and also does a good job of explaining why HTML5 is as it is. Further information can be found in the Resources link.
This may be a bit confusing if you are new to HTML: HTML5, in many cases, gives you several options for writing a tag. With the break tag <br>, for example, you may write the HTML form <br> or you may include a self-ending / as in <br/> since <br> has no closing tag. Or, you may write the proper XHTML version <br />, with the space. These pages lean more towards the XHTML versions where a choice is possible.
[Although these pages explain HTML5, some are written in XHTML. Sorry about that.]