The correct XML declaration is `<?xml version='1.0'?>` and must appear at the top of the file.
XML tags are **case-sensitive**; `<Title>` and `<title>` are considered different.
Tag names cannot contain **spaces**. Valid characters include letters, digits, hyphens, and underscores.
Empty elements must be self-closed like `<note />`.
Comments in XML are written as `<!-- comment -->`.
Every XML document must have **exactly one root element**.
A well-formed XML document follows correct syntax, such as proper tag nesting and closing.
Attributes in XML use the syntax `key='value'`, like `<book title='XML Basics'>`.
Mismatched tags violate XML syntax and result in a **parsing error**.
Special characters like `<`, `>`, and `&` must be escaped using **entity references**, e.g., `<`, `>`, `&`.