<html lang="en-US"> Is not just an opening <html> tag. It tells the natural language of the web page to the browser. It aids accessibility by helping assistive technologies like screen readers to choose correct voice profile and accent.
https://www.youtube.com/watch?v=ox5QVbZSPBk
Notice how it changes the accent and voice profile? For more information, Adrien Roselli has penned a fantastic article on the attribute lang. Do read it.
