expert from The essential guide to dreamweaver cs3 with css, ajax and php
（仅供个人学习使用~）
All mainstream modern browsers as far back as Internet Explorer 5.0 (Windows only),
Mozilla/Firefox 1.0, Netscape 7, Safari 1.2, and Opera 7.6 support the level of DOM manipulation
required by Ajax but with one important condition: JavaScript must be enabled.
This may not seem to be a major obstacle, but published statistics seem to indicate that
the proportion of people browsing with JavaScript disabled has consistently remained in
the region of 10 percent for several years 
(www.w3schools.com/browsers/browsers_
stats.asp). Many developers dispute these figures, contending that the figures are almost
certainly distorted by search engine spiders, which don’t use JavaScript.
Hopefully, the last point rang alarm bells in your head. Since search engine spiders don’t
use JavaScript, they can’t index any content or links on your site that rely on Ajax—or,
indeed, any other client-side script. You should implement Ajax with care and ensure that
your site remains navigable and meaningful even with JavaScript turned off. Fortunately,
many aspects of Spry do leave your content accessible, but Spry data sets are more of a
problem, as I’ll demonstrate later in the chapter.
See http://en.wikipedia.org/wiki/Ajax_%28programming%29#Pros_and_cons for a good
summary of the pros and cons of Ajax.
