⟩ Please explain what does DOCTYPE mean?
The term DOCTYPE tells the browser which type of HTML is used on a webpage. In turn, the browsers use DOCTYPE to determine how to render a page. Failing to use DOCTYPE or using a wrong DOCTYPE may load your page in Quirks Mode. See example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">.