Recently I've helped a few developers who have been having some weird JavaScript issues (both when using ASP.NET AJAX and with some other custom JavaScript routines they were using). The culprit was that they had automatically migrated a VS 2003 Web Project to VS 2005, and still had the <xhtmlConformance mode="Legacy"/> switch configured within their web.config file.
If you are writing custom client-side JavaScript in your web application and/or are going to be using AJAX, please read-on to learn how to avoid a common gotcha (note: for a list of other tips, tricks, recipes and gotchas I've previously posted, please check out this page here).
Symptom:
You see strange behavior when adding new client-side JavaScript to a project that was previously upgraded (successfully) from VS 2003 to VS 2005. When using ASP.NET AJAX UpdatePanel controls, this strange behavior can sometimes include cases where the page does a full-page postback instead of just incrementally updating pieces of a page.
When you open up your web.config file, you also see a <xhtmlConformance/> element within it like so:
<system.web>
<xhtmlConformance mode="Legacy" />
</system.web>
</configuration>
Background:
ASP.NET 1.0 and 1.1 didn't emit XHTML compliant markup from many of its server controls. ASP.NET 2.0 changed this and by default emits XHTML compliant markup from all controls (note: you can learn more about ASP.NET 2.0's standards compliance from this excellent MSDN article).
One of the things we noticed in the early ASP.NET 2.0 betas, though, was that when upgrading customer applications a lot of the applications had assumptions that the page output was not XHTML compliant. By changing our default output of the server controls to be XHTML, it sometimes modified the visual rendering of a page. For backwards compatibility purposes we added the <xhtmlConformance> switch above that allows developers to render controls in "Legacy" mode (non-XHTML markup the same as ASP.NET 1.1) as well as Transitional mode (XHTML Transitional) as well as Strict mode (XHTML Strict).
By default when you use the VS 2003->VS 2005 Web Project Migration wizard (for both web sites and web application projects), your web.config file will have the legacy switch added.
Solution:
Unless you know of known issues that your site has when running in XHTML mode (and which you don't have time yet to fix), I'd always recommend removing the <xhtmlConformance> section from your web.config file (or you can explicitly set it to "Transitional" or "Strict").
This will make your HTML output standards compliant. Among other things, this will cause the HTML from your server controls to be "well formed" - meaning open and close tag elements always match. This is particularly important when you are using AJAX techniques to dynamically replace the contents of certain HTML elements on your page (otherwise the client-side JavaScript sometimes gets confused about container elements and can lead to errors). It will also ensure that ASP.NET AJAX works fine with your site.