This helps browsers to display a web page correctly. The web uses different document types, so when browsers know which type and version the document was made with, then the webpage most likely won't look "strange, and quirky".
The doctype declaration (or Document Type Declaration) is not case sensitive. All cases are acceptable:
<!DOCTYPE html>
<!DOCTYPE HTML>
<!doctype html>
<!Doctype Html>
The Document Type Declaration is also known as DTD.
