If the natural language of the presentation uses a certain dialect of the language, it is possible to specify this by adding a country code as a suffix to the language code.
In this example, the natural language of the presentation is american english:
<html lang="en-us" xml:lang="en-us" >
<head>...</head>
[...]
In this example, the natural language of the presentation is british english:
<html lang="en-gb" xml:lang="en-gb" >
<head>...</head>
[...]
The country and language codes must always by separated by a hyphen.
Note that the codes for language and country are case insensitive.
A commonly used case combination for the codes is lowercase for the language code, and uppercase for the countrycode, that is:
<html lang="en-GB" xml:lang="en-GB" >
<head>...</head>
[...]
Country codes are available in Country Code Listing.