Legacy XhtmlConformance Kills Partial Page Updates

I’m writing this knowing full well that someone, somewhere will bang his/her against the wall trying to decipher one of these JavaScript errors:

Sys.WebForms has no properties

or

Sys.WebForms.PageRequestManager is null or not an object

In my case, the innocuous-looking Sys.WebForms.PageRequestManager.getInstance() was causing the latter. A big thank-you goes to Stefan Goßner and Scott Guthrie for pointing out how to fix this predicament and get partial rendering back:

<xhtmlConformance mode="Transitional"/>

<xhtmlConformance mode="Strict"/>

These empirical, anecdotal edge cases make me want to scream.