The best quick reference guide to HTML is the W3Schools tag guide. It is really just a list of all the possible HTML tags, with details on what they do and which attributes can be used with them. The latest version conforms to the HTML 5.3 standard, which is recommended for compatibility with most browsers.

There's a comprehensive style-guide, created by Tim Berners-Lee (the inventor of HTML), which is a little out of date, but of immense historical interest.

Beyond this, almost all you need to know about web editing and design can be found at the W3Schools & WebMonkey sites.