What does a "doctype" declaration inform a web browser about?

A "doctype" declaration informs a web browser about the version of HTML being used in the document. This is crucial because it helps the browser to render the web page correctly by interpreting the document according to the specified HTML version. Different versions of HTML can have varying rules and features, and without an accurate doctype, browsers may guess the document type, potentially leading to inconsistent rendering and functionality issues.

The doctype is typically the first line of an HTML document and specifies which rules the document follows, allowing browsers to switch to the appropriate rendering mode (like standards mode or quirks mode). Understanding the correct version of HTML ensures that features specific to that version are supported and displayed as intended.