Final answer:
The <html> element must have a language attribute, known as the lang attribute, which defines the primary language of an HTML document's content. This is important for assistive technologies to properly interpret the language for the user.
Step-by-step explanation:
The HTML element that must have a language attribute is the <html> element. This is specified using the lang attribute, which helps define the primary language of the document content. It's crucial for accessibility reasons, as screen readers and other assistive technologies use this attribute to provide the appropriate language support to users.
For example, to specify English, you would use <html lang="en">. While it's important to note that other elements can also have language attributes to indicate changes in the language of part of the content, the lang attribute in the <html> element pervasively sets the base language for the entire document.