They're supposed to be lower-case
Uh, no. HTML4 and HTML5 tags are case-insensitive. XHTML is case-sensitive by virtue of being based on XML rather than SGML, though. XHTML of course being a complete waste of everybody's time anyway for reasons I won't get into. Of course since it's case-insensitive it doesn't really matter which one anybody uses, though lower-case seems to be the new favourite. It's not "supposed" to be lower-case, though.
The browser's fault for not providing better accessibility
Except the fact is that by virtue of having multiple frames of content it's impossible to actually make it accessible in a consistent manner, since it multiplies the amount of content and there isn't any information available to any accessibility mechanism over what it is. Is that frame a navigational bar, or does it host the main site content? Is this frame a pointless header? etc. Without hinting like that it would be impossible for the browser to provide meaningful accessibility, but the only way to add that would be to implement non-standard tags and tag attributes within the browser, meaning it would only work on that browser.
Or, on the other hand, the site author could save everybody time and grief by not using HTML tags that should be left in 1996 where they belong, along with their relatives <BLINK> and <MARQUEE>. And the browser developers can work on implementing something more useful.
and the site's fault for not providing a fall-back option for those in need of said accessibility.