<!DOCTYPE html>
is an instruction about the version of HTML
<html lang="en">
specifies the language of the element's content
<meta charset="UTF-8">
tells the browser to use the utf-8 character encoding
<meta http-equiv="X-UA-Compatible" content="IE=edge">
ensures that IE uses the latest available rendering engine to render a web page
<meta name="viewport" content="width=device-width, initial-scale="1.0">
sets the initial zoom level when the page is first loaded by the browser