Replace cotter pin. It looks like HTML, but it has no presentation data. That comes from another source, such as a style sheet. It s like having your word processor import addresses or names via mail merge rather than typing and formatting them directly in the document. Many systems merge XML and style data back into a presentational language such as HTML for easier reading.
Browsers ignore HTML commands they fail to understand.
If items aren t properly nested, it s no big deal to the browsers. But in XML, an improperly formatted file creates a fatal error. Applications will refuse to process the file. That means a document must be what XML experts call well-formed to work right. It has to be ready for a computer program to read, and thus ready to be used in multiple ways for network delivery.