A web browser must know what language a document is written in before they can process the contents of the document.
You can let web browsers know that you are using the HTML language by starting your HTML document with a document type declaration.
The declaration is the following:
This declaration is an instruction. It tells the browser what type of document to expect, along with what version of HTML is being used in the document. must be the first line of code in all of your HTML documents.