|
About the Internet About Web Browsers Why Domain Names HTML vs XHTML Making Webpage Files Naming Webpage Files About HTML Tags Basic HTML Page DTDs and Doctype Tags Spaces and New Lines Special Characters Bold, Italics, More Writing Headlines Adding Links Making Lists Comments in HTML How to Add Images Sources of Images Image File Formats Optimizing Images Color in HTML & CSS "Web-safe" Color Chart Making Tables Formatting with Tables Making Forms Using Imagemaps Using Frames Meta Tags Intro to CSS Ways to include CSS Some Useful CSS CSS Hover for Links Promoting Your Site How-To's Homepage Links |
!DOCTYPE and DTDsHow to Use !DOCTYPE tag The tag that declares a page to be XHTML is: Strictly speaking a page is NOT an XHTML page unless an XHTML !DOCTYPE tag is the first line in the file. The tag given above is the Transitional version. Using this tag lets you use some of the older HTML features that have been superceded by XHTML and CSS if you need them for backwards compatibility. If your page has only clean XHTML coding you can use the Strict !DOCTYPE tag: !DOCTYPE tags are case sensitive. Copy them exactly as specified. What is a !DOCTYPE tag forThe purpose of !DOCTYPE tags is to let browsers and HTML validators know what type of document this is and to treat it accordingly. This is an idea that comes from eXtensible Markup Language (XML). XML is a set of rules for defining markup languages that use <tags>. HTML is an XML-like markup, but not as strict. XHTML is a proper XML version of HTML. Each XML language has a Document Type Definition (DTD). A DTD is a computer file containing the definitions of all the tags and how they can be used. The !DOCTYPE tag tells where this .dtd file is, by including its URL. http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd for the transitional one. To learn to read and write DTDs - read about XML or take an XML class. HTML also can use !DOCTYPE tags, though they aren't generally required. For example the !DOCTYPE tag for HTML 4.0 transitional is Unless you tell them to expect something else, web browsers do expect your page to be HTML. So HTML and XHTML web pages without !DOCTYPE tags will still work. In order to validate your code - that is let a program check it for errors - you will need to specify a doctype. The newest browsers (version 6+ of Netscape and IE) read doctypes and parse documents differently depending on the doctype. If you use a full doctype with URL they will sometimes be stricter than expected. Older browsers just ignore DTDs. This doctype switch table by Matthias Gutfeldt shows how the browsers that understand doctypes respond to the various doctype declarations. He also explains a bit about why the browser makers did it this way. He has links to more doctype info in that article also. |
See Also:
