The Motive Web Design Glossary
document type definition (DTD)
The DTD affects the way a webpage displays in different browsers. This is particularly noticeable when setting type sizes and also affects the padding and spacing around elements.
The document type definition (DTD) is the validation standard used by a browsing device when it displays a webpage. The definition specifies the code base used (language) by the webpage (and consequently the elements that are permitted/valid).
Using a linguistic analogy; the DTD is comparable to a dialect. Some words (elements) may be shared amongst dialects, but others will be unique to a single dialect. The ‘big ideas’ are readily understood, but the nuance may be lost. In the case of displaying a webpage, this nuance may relate to the size of the type, space around form elements, etc.
DTD interpretation: strict, transitional, frameset
Due to the constant evolution of standards, DTDs allow for a degree of flexibility. This flexibility is referred to as ‘interpretation’ and can be set to one of three values;
- Strict: only code that is pure/correct is used, anything obsolete/deprecated will not be interpreted
- Transitional, or loose: allows backward compatibility between the current and older code standards
- Frameset: used for frameset validation
document type declaration (doctype)
The document type declaration is often referred to as the ‘doctype’.
The DTD is specified in a document type declaration, the formal ‘presentation’ of the chosen DTD. The declaration is made before the content of the webpage.
Sample document type declaration, specifying a (strict) DTD:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
"http://www.w3.org/TR/REC-HTML40/strict.dtd" />
Thanks to Greg Feldman for ‘ironing out the kinks’ in our description
deprecated, frames/frameset, quirks mode, elements,tags, validation
References and further reading
- The art of reading a DTD (Autistic Cuckoo)
Making sense of DTD notation and structure; comments, entities, elements and attributes.- Doctype declarations and content-type headers
Although the code base used to author a webpage is declared by the doctype, thecontent-typeis communicated to the browser by acontent-typeheader, i.e. thecontent-typeis set by the server. Although a doctype may specify XHTML, thecontent-typemust also be set toapplication/xhtml+xml, to correctly serve the document. Unfortunately Internet Explorer 6 does not correctly interpret thexhtml+xmlcontent-typeheader…
- Doctype declarations and content-type headers
- Fix your site with the right doctype! (A List Apart)
Why the doctype is important. Quirks mode. List of valid doctypes. - Picking a rendering mode (Eric Meyer)
The impact of rendering modes on CSS. - The global structure of an HTML document (W3C)
- Use the right doctype (Eric Meyer, O’Reilly Network)
Introduction and which doctype definitions will trigger quirks mode in IE Mac.
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