Spacer image should have a blank alt attribute to meet w3c accessability standards

Our User Experience Design team have raised an issue with the use of the tapestry.spacer-image in the Autocomplete mixin. It's also used by the default client side validation. The issue is that the spacer img tag is rendered with no alt attribute which is apparently a problem for these reasons:

1) Always add alt attributes to images. Alt is mandatory for accessibility and for valid (X)HTML.
2) For images that play only a decorative role in the page, use an empty alt (i.e. alt="") or a CSS background image so that reading browsers do not bother users by uttering things like "spacer image".

So, Tapestry needs to write a blank alt attribute when it uses a spacer gif.