In biology, cell theory is a scientific theory which describes the properties of cells. These cells are the basic unit of structure in all organisms and also the basic unit of reproduction.
The doctype declaration should be the very first thing in an HTML document, before the <html> tag. The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in. The doctype declaration refers to a Document Type Definition (DTD).
HTML Validator. Validates the HTML string/file for well-formedness and compliance with w3c standards. It tries to make use of the doctype declaration to evaluate the document structrue but will resort to best practice if unmatched.he validator will report on missing or invalid attributes, unknown tags, unclosed tags and more. *This validator does NOT support HTML 5! I leave it here for backward validation, but if you need a HTML 5 validator, try the following site HTML 5 - Validator.Nu.