What is "doctype"

DOCTYPE (short for Document Type Definition - Document type definition) serves to define how browsers will behave when reading your code. This statement should always appear at the beginning of every document you create (even before the tag ).

There are some types of documents you can declare. In HTML 4.01 and XHTML 1.0 we will find 3 type, Strict (no errors allowed structure and use of CSS is mandatory), Transitional (more flexible, allows browsers to navigate without CSS support) and Frameset (used for documents that use frames). Already in HTML 5 and XHTML 1.1 we will find only one option.

Link to Doctype types: Link