Doctypes - short for 'document type' - help browsers to understand the version of HTML the document is written in for better interpretability. Doctype declarations are not HTML tags and belong at the very top of a document. This topic explains the structure and declaration of various doctypes in HTML.
Adding the Doctype
The <!DOCTYPE> declaration should always be included at the top of the HTML document, before the <html> tag.
Version ≥ 5
See HTML 5 Doctype for details on the HTML 5 Doctype.
HTML 5 Doctype
HTML5 is not based on SGML (Standard Generalized Markup Language), and therefore does not require a reference to a DTD (Document Type Definition).
HTML 5 Doctype declaration:
Per the W3.org HTML 5 DOCTYPE Spec:
A DOCTYPE must consist of the following components, in this order:
1. A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".
therefore the following DOCTYPEs are also valid:
This SO article discusses the topic extensively: Uppercase or lowercase doctype?
HTML provides not only plain paragraph tags, but six separate header tags to indicate headings of various sizes and thicknesses. Enumerated as heading 1 through heading 6, heading 1 has the largest and thickest text while heading 6 is the smallest and thinnest, down to the paragraph level. This topic details proper usage of these tags.
Headings can be used to describe the topic they precede and they are defined with the <h1> to <h6> tags. Headings support all the global attributes.
- <h1> defines the most important heading.
- <h6> defines the least important heading.
Defining a heading:
Correct structure matters
Search engines and other user agents usually index page content based on heading elements, for example to create a table of contents, so using the correct structure for headings is important.
In general, an article should have one h1 element for the main title followed by h2 subtitles – going down a layer if necessary. If there are h1 elements on a higher level they shoudn't be used to describe any lower level content.
Example document (extra intendation to illustrate hierarchy):