This rule is defined by the following XPath expression:
//Element[string:upper-case(@Name)="IFRAME"][count(Attribute[string:upper-case(@Name)="SRC" ]) = 0]
Example:
<HTML><title>bad example><BODY> <iframe></iframe> </BODY> </HTML> <HTML><title>good example><BODY> <iframe src="foo"></iframe> </BODY> </HTML>