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).
The <style> tag is used to define style information for an HTML document. Inside the <style> element you specify how HTML elements should render in a browser. Each HTML document can contain multiple <style> tags. Tip: To link to an external style sheet, use the <link> tag.