Doctype checker
What is a Doctype Checker?
The Doctype Checker reports the DOCTYPE declaration at the very top of a webpage's HTML source, which tells the browser which version of HTML the page is written in and how to render it.
Why the Doctype Matters
- Rendering mode — A missing or incorrect doctype can trigger "quirks mode" in browsers, where legacy, inconsistent rendering rules from the 1990s are applied instead of modern standards mode.
- HTML5 simplicity — The modern HTML5 doctype is simply
<!DOCTYPE html>, far shorter than the verbose DTD declarations required by XHTML/HTML4. - Cross-browser consistency — Standards mode (triggered by a correct doctype) is what makes CSS behave consistently across different browsers.