In the childhood of Web design (1995) it was necessary to learn HTML - HyperText Markup Language - to create Web pages. These were hand coded in poor and plain text editors. In addition, the HTML was evolving at a fast pace and each browser implemented its own flavour of HTML.

  Today, it is possible to create magnificent web sites without knowing any HTML, thanks to the Web publishing packages. In the meantime HTML has also stabilised. Notwithstanding, I leave here some links to the readers interested in "how it works".

  There are thousand of references on introduction to HTML. It would be presomptious of mine to indicate the best references. These are the ones I like the most from all those I read.
  Two notes:

1. Try to learn HTML 4. There are significative enhancements from previous versions and also many tags that will be unsupported in the future although currently in use. HTML 4 widens the possibilities of Web publishing and simplifies some features.
2. Avoid references that just present the command followed by an example. In this manner you'll never learn HTML; at most you'll clarify your doubts on what you learned by other sources.
    

Teach Yourself Web Publishing with HTML 4 in 14 days by Laura Lemay (book) - It's a comprehensive reference that guides the reader from its first steps to a medium level of competence.
That's how I started in March 1997...

HTML Tutorial - A HTML 3.2 manual by Jukka Korpella from Finland with additional pages on web publishing.

  The W3C - World Wide Web Consortium defines the language specifications to ensure compatibility and accessibility of he contents to all platforms. These references are the HTML grammar: they assist you to write correctly by they don't teach you how to write.

  In case of doubt, see the service http://www.htmlhelp.com maintained by the Web Design Group for more user-friendly versions of the norms.
 

HTML 3.2 specification - The previous standard of HTML, implemented by the 3.0 browsers. The Web Design Group (WDG) offers versions in various formats for download. These are very confortable to use off-line as an assistant or a grammar.

HTML 4.0 specification - The current standard. A comprehensive definition but too long for most readers. There is a variant prepared by the WDG.

CSS 1.0 specification - The Cascading Style Sheets (CSS) are the correct form of formatting the elements in a Web page. The 1.0 specification is implemented by 4.0 browsers. O WDG offers a variant for download.

CSS 2 specifications - The new specification recommended from 1999 onwards, but the 2000 browsers implement it only partially. This is a comprehensive and long reference, perhaps too arid for beginners.

HTML Tidy. Free application to verify the correction of HTML (for MS Windows).

Bobby - Accessibility validation with emphasis on the users with special needs. Submit your pages and you will receive advices on improvements. The test may be performed on line or on your computer using a free program (for MS Windows).