One of the casualties of 20th century tag soup was the structure of an HTML document. Since the default look of many HTML elements like headings, lists and paragraphs was not what designers wanted, and browser support for styling these elements with CSS was lousy, many people chose to remove them from their markup.
Now that browsers have matured and have good support for styling basic HTML elements, many people still don’t use the available, proper markup. I guess it’s because structure has been forgotten. The only thing that counts to some is how the result looks. That is why the web is full of code like <span class="headerstyle1">this is my header</span> and <span class="boldtext">this is bold</span>.
Some great resources for refreshing your knowledge of basic XHTML are:
- Semantics, HTML, XHTML, and Structure by Shirley E. Kaiser
- Standards-compliant XHTML: Introduction to eXtensible HyperText Markup Language (XHTML) by Dave Rau