Ajax can be a really powerful tool if used properly. A common mistake in Ajax implementation is the lack of a fallback mechanism to be deployed when Ajax support is not present or partial. A common scenario in that sense is made up by text browsers and screen readers.
onfocus event on a text input, for example when you have to display some warnings during form validation. Here's how you can implement a fallback mechanism:
<input type="text" name="email" id="email" /> <noscript> <p>The email format must be in the firstname.lastname@example.org format.</p> </noscript>
By doing so, the message will be available even to text browsers that, in addition, don't support CSS neither so that you cannot hide text with CSS properties.
In a nutshell, the best thing to do is to implement a solution like the one mentioned above instead of relying on a support that can be very poor or absent.