The purpose of the doctype declaration in HTML5 is to specify the version of HTML being used in the document and to ensure that the browser renders the webpage in standards mode. The doctype declaration is placed at the very beginning of an HTML document, before the `` tag. In HTML5, the doctype declaration is simplified and standardized. The following is the basic syntax of the doctype declaration in HTML5:

<!DOCTYPE html>

The above declaration informs the browser that the document is written in HTML5. It triggers the browser to render the webpage in standards mode, where it follows the specifications and rules of HTML5.

<!DOCTYPE html>
  <title>My Webpage</title>
  <h1>Welcome to My Webpage</h1>
  <p>This is a sample paragraph.</p>

In this example, the `<!DOCTYPE html>` declaration is placed at the very beginning. It signifies that the document is an HTML5 document. The rest of the HTML markup follows after the doctype declaration. Conclusion : It's important to include the <doctype declaration> to ensure proper rendering and interpretation of the HTML code by web browsers. Without it, the browser might fallback to quirks mode or older rendering modes, leading to inconsistent behavior and potential compatibility issues.