Check for ARIA Landmarks

[Heydon]

Some examples:

Checking for role="main" /

on #main...

#main:not([role="main"]):not(main):after {
   content: 'error message';
}

Checking for role="banner" on page-level header:

body > header:not([role="banner"]), 
#header:not([role="banner"]), 
#page-header:not([role="banner"]) {
   content: 'error message';
}

You could do the same for contentinfo on the page footer, maybe?