Understanding HTML Document Types and Their Impact
Learn about the importance of the Document Type Declaration (DOCTYPE) in HTML. This tutorial explains the simple HTML5 DOCTYPE and contrasts it with the more complex declarations used in older versions of HTML (HTML 4 and XHTML). Discover which HTML elements are valid across different DOCTYPEs.
The HTML Document Type
Every HTML document needs to start with a document type declaration. This isn't an HTML tag itself; instead, it's information that tells the web browser what type of document it's dealing with.
HTML5 Document Type Declaration
In HTML5, the declaration is very simple:
Syntax
<!DOCTYPE html>
Older Document Types (HTML 4 and XHTML)
Older HTML documents (HTML 4 or XHTML) use a more complex declaration because it needs to reference a Document Type Definition (DTD). The DTD specifies the rules and structure of the document.
You can find more details about different document types in the HTML reference (replace with actual link).
Valid HTML Elements Across Different DOCTYPES
The table below shows whether various HTML elements are valid in HTML5, HTML 4, and XHTML. "Yes" means the element is supported; "No" means it's not.
| Tag | HTML 5 | HTML 4 | XHTML |
|---|---|---|---|
| <a> | Yes | Yes | Yes |
| <abbr> | Yes | Yes | Yes |
| <acronym> | No | Yes | Yes |
| <address> | Yes | Yes | Yes |
| ... | ... | ... | ... |
Note: Due to the extensive list, only a few example rows are shown. Please complete the table with the remaining tags and their compatibility.