HTML <!DOCTYPE> Declaration
Having trouble with your Web pages? Can't get them to display correctly in your browser? It might be that your page is a little "quirky."
There are several reasons why Web pages do not display properly, such as omitting a required end tag, improperly formatting a table, JavaScript errors, CSS errors, incorrect server requests, etc. Often, these errors happen because Web developers do not take the time to validate their work. In other cases they are a result of a lack of knowledge regarding the proper formatting and coding of Web pages.Even if you take care of all these errors, something else may be causing your Web pages to display incorrectly or even not at all: failure to use a
<DOCTYPE> statement or, if used, failure to use the right one.<DOCTYPE> statement (Interestingly, many of them are Web development sites). Does it really make a difference? For the most part, not much. But if you want to make sure your Web site renders correctly in various browsers, then it's best to use it. You'll also need it if you plan on validating your Web pages.What Is It?
The<DOCTYPE> declaration tells the browser what version of (X)HTML is being used so it will know how to display the page. The declaration may also provide a link to a text file (i.e., the loose DTD) which the browser uses to obtain further information. It's similar to having a tune-up done on your car. In order to perform it correctly, the mechanic must know the make, model, and year of the car. He must also know how many cylinders the engine has and its horsepower. Once he knows this, he can move forward and properly tune the engine.Usage
The<DOCTYPE> declaration must be the very first thing in your document, before the <html> tag. Then, when the browser begins to load the page, it knows how to treat the code within the page.There are three different document types for HTML 4.01: Strict, Transitional, and Frameset. The Strict type is used with Cascading Style Sheets and helps to ensure clutter-free coding.
The Transitional type is used when HTML presentational features are included in the document instead of in a style sheet. This is done to accomodate older browsers that don't support CSS.<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
The Frameset type is used in documents that have frames.<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
The same three document types are also used in XHTML 1.0:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
<DOCTYPE> declaration is actually a comment tag, it won't confuse older browsers that don't understand the statement.
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