The better you code, the more efficient, flexible, consistent your site will be. Consult http://www.w3.org for guidelines and standards.
One of the most important recommendations of HTML 4.0 is the separation of structure and style.
With style sheets, you can separate the structure of a web page from its design. As a result, you can control a document’s presentation in ways HTML can’t.
Deprecated refers to those HTML tags and attributes that should be phased out in favor of style sheets.
Good pages to look up depracated tags http://www.codehelp.co.uk/html/deprecated.html and
Is it a good idea to avoid using deprecated tags?
What does it mean to separate style and structure, and why is it important?
Means to define the presentation of structural elements outside the code as often as possible. This is done with CSS.
Advantages of CSS
XHTML (Extensible hypertext markup language)
XHTML is a strict version of HTML that shares the requirements of well-formedness with the “meta-languages” XML (Extensible Markup Language) and SGML (Standard Generalized markup language).
Advantages of XHTML
Because of the future hand held devices, appliances and other electronics will make more use of the internet.