About the Internet
About Web Browsers
Why Domain Names
About HTML Tags
Basic HTML Page
DTDs and Doctype Tags
Spaces and New Lines
Bold, Italics, More
Comments in HTML
How to Add Images
Sources of Images
Image File Formats
Color in HTML & CSS
"Web-safe" Color Chart
Formatting with Tables
Intro to CSS
Ways to include CSS
Some Useful CSS
CSS Hover for Links
Promoting Your Site
HTML vs XHTML
XHTML - eXtensible HyperText Markup Language is poised to replace HTML - plain old HyperText Markup Language the original core language which is the heart of every web page.
XHTML is really HTML forced to conform to XML (eXtenisible Markup Language) rules. XML is commonly used in web applications and services. The strict rules allow better automation of data processing, but even with plain static webpages, adherence to the rules makes it more likely that the page will display correctly in all browsers.
If you are just learning web design then do it right and start with XHTML. If you already know HTML don't worry - you almost know XHTML. You may have a few old coding habits to break and a few new ones to learn that's about it.
The average web surfer will not notice any difference between XHTML and HTML pages. Older browsers should have no problem with XHTML, to them it just looks like stricter, well-formed HTML.
On the other hand for newer browsers and other devices, including pdas, cell-phones, and screen readers for the blind using XHTML increases compatibility and allows them to better interpret web pages and make them more accessible. This is good news if you care about reaching your full potential audience.
XHTML requires a doctype to be specified. See DTD article in HTML Basics.
All tags and attributes must be written in lower case. All attribute values must have quotes around them, either " or ' will work as long as you use the same character to end the quote as you did to start it. See Tags article in HTML Basics.
All tags must be closed, even non-container ones like img, br, and hr. There is a special shorthand for this. So instead of having to write <br></br> you can simply write <br />. As long as you put the blank space between the end of the tag and the closing /> older browsers have no trouble understanding this.
Tags who's closing was optional in HTML, but that contain data are closed as you should expect. For example a list item would properly be <li>Some Item </li>.
All tags must be properly nested, but this has always been part of the rules even if some browsers have been lax about it.
Use Cascading Style Sheets (CSS) to format your text. The font tag is gone from XHTML. CSS for things like text size and colors is easier to maintain and update than font tags anyways, so rejoice.
Use of CSS for positioning page layouts instead of using tables to do so is increasing, and often vehemently recommended. It has its benefits and its headaches. I am still deciding on a case-by-case site-by-site basis how much of each to use.