Don't forget to add a doctype
add what?
There is not just one type of HTML, there are actually many: HTML 4.01 Strict, HTML 4.01 Transitional, XHTML 1.0 Strict, and many more. All these types of HTML are defined in their respective W3C specifications, but they are also defined in a machine-readable language specifying the legal structure, elements and attributes of a type of HTML.
Such a definition is called a "Document Type Definition", or, for short, DTD.
Tools which process HTML documents, such as Web browsers, need to know which DTD an (X)HTML document is actually using: this is why each (X)HTML document needs, at the beginning, a DTD declaration, such as:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Given its syntax, the DTD declaration is often called, for short, "Doctype".
Why?
Why specify a doctype? Because it defines which version of (X)HTML your document is actually using, and this is a critical piece of information needed by browsers or other tools processing the document.
For example, specifying the doctype of your document allows you to use tools such as the Markup Validator to check the syntax of your (X)HTML (and hence discovers errors that may affect the way your page is rendered by various browsers). Such tools won't be able to work if they do not know what kind of document you are using.
But the most important thing is that with most families of browsers, a doctype declaration will make a lot of guessing unnecessary, and will thus trigger a "standard" parsing mode, where the understanding (and, as a result, the display) of the document is not only faster, it is also consistent and free of any bad surprise that documents without doctype will create.
Further Reading
- Fixing your site with the right Doctype, an article on A List apart by Jeffrey Zeldman
- a list of valid doctypes for (X)HTML documents
About the "QA Tips"
The W3C QA Tips are short documents explaining useful bits of knowledge for Web developers or designers, hosted and produced by the Quality Assurance Interest Group at W3C.
While the tips are carefully reviewed by the participants of the group, they should not be seen as anything else than informative bits of wisdom, and especially, they are not normative W3C technical specifications.
Learn more about the Tips, how to submit your own pearls of wisdom, and find all the other QA tips in the Tips Index.