XML Prolog and Declaration

Estimated study time: 8 minutes.

Most XML documents begin with a special line called the prolog, which contains the XML declaration. It isn't a tag in the usual sense — it's metadata that tells the parser how to interpret the document.

The Declaration Line

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

Breaking It Down

• version — the XML specification version being used, almost always 1.0.
• encoding — the character encoding of the document, commonly UTF-8.
• standalone — indicates whether the document relies on an external DTD ("no") or is self-contained ("yes").

Placement Rules

• The declaration must be the very first line of the document — nothing, not even whitespace, can come before it.
• It's technically optional, but strongly recommended for portability and correct parsing.

What Goes in the Prolog Besides the Declaration?

The prolog can also include comments and a Document Type Declaration (DTD) reference, all placed before the root element.

<?xml version="1.0" encoding="UTF-8"?>
<!-- Course catalog data -->
<!DOCTYPE course SYSTEM "course.dtd">
<course>...</course>