The more immediate problem with Tim Berners-Lee's choice of delimiters in URL syntax is that ampersand starts an entity reference in SGML default concrete synta

The more immediate problem with Tim Berners-Lee's choice of delimiters in URL syntax is that ampersand starts an entity reference in SGML default concrete syntax and thus <a href="bla&x=y"> will be rejected as a reference to an undeclared entity "x" in SGML (whereas in XML it will be rejected as incomplete entity reference missing a terminating ";" character).