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L'attributo language del tag script è deprecato in HTML e non presente nella DTD XHTML Strict.

Dalle specifiche HTML W3C:
language = cdata [CI]
Deprecated. This attribute specifies the scripting language of the contents of this element. Its value is an identifier for the language, but since these identifiers are not standard, this attribute has been deprecated in favor of type.
Dal sito wc3schools:
Differences Between HTML and XHTML
The "language" attribute of the script element was deprecated in HTML 4.01.
The "language" attribute of the script element is not supported in XHTML 1.0 Strict DTD.