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However, whether this makes any practical difference to how current browsers or search engines handle the page is open to debate.
Current W3C specs say to declare your language in the HTML element, directly after a valid DOCTYPE declaration:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
Use "http-equiv" for instructions to the browser application, itself.
i.e.
<meta http-equiv="refresh" content="1; url=page.html">
Use "name" for instructions to other applications.
i.e.
<meta name="keywords" content="search,terms,here">
"http-equiv" causes something to change in the default browser behaviour, where "name" provides information and does not cause a change in behaviour.
Also, HTML is case-insensitive. If it's in an HTML tag (like META), and not script-related, it doesn't matter if you use upper- or lowercase. I tend to use all lowercase, because that's one less key I need to strike during coding.
<wincing in preparation for a stomp> :)