Attributes should be lower case in XHTML (e.g.http://www.w3.org/TR/xhtml1/diffs.html#h-4.2).
Concordion is able to parse attributes with upper case letters such as assertEquals, but also in lower case such as assertequals.
All of these will work:
<span concordion:assertEquals="#name">Fred</span>
<span concordion:assert-equals="#name">Fred</span>
<span concordion:assertTrue="#name">is true</span>
<strong concordion:assert-true="#name">is true</strong>
<div concordion:assertFalse="#name">Fred</div>
<p concordion:assert-false="#name">Fred</p>