The subheader is rendered ithin the header as {subheader}. If you want to restore the FSC / HTML5 <> construct, remove "{subheader}" and comment in the partial "subheader" in partial "all".

{f:if(condition:'{subheader}', then:'{subheader}')}{header}

{f:if(condition:'{subheader}', then:'{subheader}')}{header}

{f:if(condition:'{subheader}', then:'{subheader}')}{header}

{f:if(condition:'{subheader}', then:'{subheader}')}{header}

{f:if(condition:'{subheader}', then:'{subheader}')}{header}
{f:if(condition:'{subheader}', then:'{subheader}')}{header}
-- do not show header --