1 from ping mag its really funny but informative.

2 from 37 signals Using patterns in webdesign

This document will summarize the level of support for web standards and maturing technologies in popular web browsers. It covers the Internet Explorer, Firefox, and Opera web browsers, with focus on the HTML, CSS, DOM, and ECMAScript technologies.

http://www.webdevout.net/browser_support.php

The meaning of alt text is to serve as replacement (hence, “alternative”) when the image the HTML intends to serve cannot be seen. There’s a variety of reasons for that to happen; the user is blind, the medium doesn’t support images, the user disabled images on purpose so she can browse on low-bandwidth, or the Googlebot comes around to visit.

For example, when your image is a mere illustration to a point you’re repeating elsewhere in the text, there’s no need at all to use the alt text. Because the image is not crucial to understanding the point. Most people however at this point heard some rumors that the “alt tag” is increasing accessibility; and possibly, they’ve heard somewhere else that accessibility is professional these days. And they also might want to see a tool-tip, not knowing that the title attribute would come in handy for that. (Wordpress in typical installations even creates an alt text based on the image’s file name; tools like Frontpage have committed similar silliness in the past.)

So, let’s say our blog post is titled “New AJAX Flavor Discovered” and the illustration following the headline is a shrink-wrap box with the shiny colorful letters “AJAX,” then repeating the letters AJAX in the alt text will result in something like this within alternative browsing contexts:

New AJAX Flavor Discovered
AJAX

Is this useful? No – the point of the illustration was to use a recognizable catchy visual, or to suggest that AJAX is hyped as a shrink-wrapped shiny product; the point was not to clarify (in text) that the article is about AJAX, as the headline already did a good job at that.

In XHTML2, by the way, some of us may be able to get rid of the alt attribute altogether. Why? In XHTML2 you can use the “src” attribute on anything, including e.g. a paragraph. It’s a bit like longdesc (today’s link to a longer image description) done right.

Misunderstand Web Standards

Most standards aware web professionals can be found here. These people tend to think about structure and semantics first and presentation later. Strict doctypes are generally used at this level to encourage the separation of semantic and presentational markup. Whether the markup language is HTML 4.01 or XHTML 1.0 is less important to many. In fact, some people at Level 5 are questioning the use of XHTML. Level 5 people tend to get into endless discussions about tiny markup details. They can waste hours thinking about class names or reorganising their HTML and CSS files to make their code look pretty.

If you have reached this level I think you’re doing extremely well. For most people there is no compelling reason to move beyond this Level 5.

Read about all the 6 level here