The Attribute
The lang attribute on the <html> tag defines the primary language of the page.
<!DOCTYPE html>
<html lang="en">
Why It Matters
Without this, a screen reader may attempt to read an English page with a French pronunciation engine (or vice versa), making it incomprehensible.
Language Parts
If a specific paragraph switches language (e.g., a quote), wrap it:
<p>The motto is <span lang="la">Carpe Diem</span>.</p>