XSL Formatting Objects documents are XML documents, but they do not have to conform to any schema. Instead, they conform to a syntax defined in the XSL-FO specification.

XSL-FO documents contain two required section. The first section details a list of named page layouts. The second section is a list of document data, with markup, that uses the various page layouts to determine how the content fills the various document pages.

The properties of the page define by the Page layout. It can define the directions for the flow of text, so as to match the conventions for the language in question. They define the size of a page as well as the margins of that page. Most important that they can define sequences of pages that allow for effects where the odd and even pages look different.Example one can define a page layout sequence that gives extra space to the inner margins for printing purposes; this allows more space to be given to the margin where the book will be bound.

The document data portion is brake up into a sequence of flow, where each flow is attached to a page layout. The flows contain a list of blocks and each contain a list of text data, inline markup elements, or a combination of the two. Content may also be added to the margins of the document, for page numbers, chapter headings and the like.

Blocks and inline element function are the same way as for CSS, though some of the rules for padding and margins differ between CSS and FO. The direction, relative to the page orientation, for the progression of inlines and blocks can be fully specified, thus allowing FO documents to function under languages that are read different from English. The language of the FO specification, unlike that of CSS 2.1, uses direction-neutral terms like start and end rather than left and right when describing these directions.

XSL-FO's basic content markup is derived from CSS and its cascading rules. Many attributes in XSL-FO propagate into the child elements unless explicitly overridden.

The <fo:root> element is the root element of XSL-FO documents.

<fo:layout-master-set> tag element contains one or more page templates

Each <fo:simple-page-master> tag element contains a single page template. Each template must have a unique name

One or more <fo:page-sequence> tag describe the page contents. The master-reference attribute refers to the simple-page-master template with the same name: