WOFF metadata and private data

After the font tables a WOFF file may carry two more blocks, both optional: extended metadata, an XML document about the font, its makers and its licence; and private data, which can hold anything. Neither is part of the font. They must not change how the font loads or looks, and a browser does not need to read them. A file without them, like the one in the header example, has zeros in all five of their offset and length fields.

Where the blocks go

The blocks of a WOFF file come in a fixed order: header, table directory, font tables, metadata, private data.

The metadata block, when present, comes straight after the last font table, which is padded to a four-byte boundary, so the block starts on one. metaOffset says where it starts and metaLength how long it is. If it is the last block in the file, no padding should follow it.
The private data block, when present, is always the last block and ends exactly at the end of the file. It starts on a four-byte boundary, with up to three zero bytes of padding after the metadata or the last table.
A block that is absent has both its offset and its length set to 0. A block that overlaps a table or the other block, runs past the end of the file, or leaves bytes that are not padding makes the whole file invalid.

Extended metadata

The metadata is an XML document in UTF-8, compressed with zlib exactly as the font tables are. Unlike them it is always compressed, even when that does not make it smaller, so metaLength is the compressed length and metaOrigLength the length of the XML.

The block is invalid if it does not uncompress, if it uncompresses to a length other than metaOrigLength, or if the XML is not well-formed, not UTF-8 or does not follow the schema below. That includes an element the schema does not list: the extension element is the place for anything else. A user agent must ignore an invalid metadata block, as if it were not there, and still load the font.

The elements are in no namespace. The root is metadata, and its children may come in any order. Elements whose content is text can be given in several languages, one text element per language.

Elements

Every element the specification defines. Required attributes are marked; the rest are optional.

Element	Inside	How many	Attributes	Content and meaning
`metadata`	root	1	`version` (required, `1.0`)	The document. Its children, all optional, in any order: `uniqueid`, `vendor`, `credits`, `description`, `license`, `copyright`, `trademark`, `licensee`, `extension`.
`uniqueid`	`metadata`	0 or 1 (recommended)	`id` (required)	Empty. An identifier for this font, whose form the maker chooses; the specification recommends a reverse-DNS prefix, such as `com.example.font.123`. It is not an XML `ID`.
`vendor`	`metadata`	0 or 1 (recommended)	`name` (required), `url`, `dir`, `class`	Empty. The font’s vendor, and a link to its site.
`credits`	`metadata`	0 or 1	none	One or more `credit` elements.
`credit`	`credits`	1 or more	`name` (required), `url`, `role`, `dir`, `class`	Empty. A person or company who worked on the font. `role` is free text, such as “Lead” or “Contributor”; no values are defined.
`description`	`metadata`	0 or 1	`url`	One or more `text`: a description of the font.
`license`	`metadata`	0 or 1	`url`, `id`	Zero or more `text`: the licence, or a summary of it. `url` links to the full licence; `id` is an identifier for it. The only element that may be empty.
`copyright`	`metadata`	0 or 1	none	One or more `text`: the copyright notice.
`trademark`	`metadata`	0 or 1	none	One or more `text`: the trademark notice.
`licensee`	`metadata`	0 or 1	`name` (required), `dir`, `class`	Empty. Who the font was licensed to.
`extension`	`metadata`	0 or more	`id`	Zero or more `name` (a title for the group, one per language), then one or more `item`. Metadata the specification does not define.
`item`	`extension`	1 or more	`id`	One or more `name` and one or more `value`: a name and value pair, in as many languages as wanted. An item without a name or a value is invalid and should be ignored.
`name`	`extension`, `item`	0 or more in `extension`, 1 or more in `item`	`xml:lang`, `dir`, `class`	Text.
`value`	`item`	1 or more	`xml:lang`, `dir`, `class`	Text.
`text`	`description`, `license`, `copyright`, `trademark`	1 or more (0 or more in `license`)	`xml:lang`, `dir`, `class`	Text in one language, which may contain `div` and `span`.
`div`	`text`, `div`	0 or more	`dir`, `class`	A block of text, such as a paragraph; may contain text, `div` and `span`.
`span`	`text`, `div`, `span`	0 or more	`dir`, `class`	A run of text inside a line; may contain text and `span`.

Common attributes

Attribute	Values	Meaning
`xml:lang`	a BCP 47 language tag, such as `en` or `fr`	The language of the text. Older files use a plain `lang` attribute instead, which readers should treat the same way; new files should use `xml:lang`.
`dir`	`ltr` or `rtl`	The direction of the text, left to right or right to left. Left to right when it is missing.
`class`	tokens separated by spaces	Names for styling or for software to use. No values are defined.
`url`	a URL	A link for the element: the vendor’s or contributor’s site, more about the font, the full licence.
`id`	text	An identifier, on `uniqueid`, `license`, `extension` and `item`. None of them is of the XML type `ID`.

Example

A small metadata document in the shape of the specification’s examples, using every kind of element:

<?xml version="1.0" encoding="UTF-8"?>
<metadata version="1.0">
  <uniqueid id="com.example.sans.regular.1-000"/>
  <vendor name="Example Type" url="https://example.com/"/>
  <credits>
    <credit name="A. Designer" role="Lead"/>
    <credit name="B. Engineer" role="Contributor"/>
  </credits>
  <description>
    <text xml:lang="en">A plain sans serif for text.</text>
  </description>
  <license url="https://example.com/licence" id="example-web-1">
    <text xml:lang="en"><div>For use on example.com only.</div></text>
    <text xml:lang="fr"><div>Pour example.com uniquement.</div></text>
  </license>
  <copyright>
    <text xml:lang="en">Copyright 2012 Example Type.</text>
  </copyright>
  <trademark>
    <text xml:lang="en">Example Sans is a trademark of Example Type.</text>
  </trademark>
  <licensee name="Example Ltd"/>
  <extension id="com.example.build">
    <name xml:lang="en">Build</name>
    <item id="tool">
      <name xml:lang="en">Tool</name>
      <value xml:lang="en">fontmake</value>
    </item>
  </extension>
</metadata>

Private data

The private data block holds whatever the font’s maker wants to put there, in any format. The specification defines nothing about its contents, and a user agent must make no assumptions about them and must not let them change how the font is used. It is not compressed by WOFF; privLength is simply its length in bytes.

Its only rules are about where it goes: it starts on a four-byte boundary at privOffset and is the last thing in the file, so privOffset + privLength equals the header’s length.