Comment 6 for bug 1002242

This syntax didn't cause compilation errors (and worked perfectly well) in the previous versions of zope that I've used running up to zope 2.13.10.

It's also how it's documented in wiki.zope.org ; e.g. in their example you're shown the correct syntax should be as follows :
<a tal:attributes="href string:${context/absolute_url}/foo"> Foo </a>

as taken from :
http://wiki.zope.org/zope2/PageTemplateStyleGuide