definitions-autolinks.md

Definitions

Defining a term is as easy as wrapping a <dfn> element around it. Most of the time, this is all you'll need to do - the definition automatically gains an id, and is usually automatically exposed as an autolink target for local and cross-spec autolinks.

Autolinking is a special mechanism in the processor to let you link terms to their definitions without having to explicitly provide a url. Instead, the text of the link is matched against the text of the definitions, and if a match is found, the link's href is set up to connect the two.

Changing the Linking Text

Sometimes, the text of the definition isn't exactly what you want it to be linked by, or you may want it to be linkable with more than one phrase. For example, an algorithm named "Check if three characters would start an identifier" may also want to be linkable from the phrase "starts with an identifier". To alter the linking text, simply add a title attribute to the definition; the title text is used instead of the text content. You can separate multiple linking phrases by separating them with the pipe "|" character.

Definition Types

All definitions have a definition type. This allows for "namespacing" of the linking text, so you can define, for example, both a property and a term with the same linking text, such as "direction" or "color".

There are several types for CSS values:

• property
• descriptor (the things inside at-rules like @font-face)
• value (any value that goes inside of a property, at-rule, etc.)
• type (an abstract type, like <length> or <image>)
• at-rule
• function (like counter() or linear-gradient())
• selector
• token

There are additional types for WebIDL definitions:

• interface
• method
• attribute
• dictionary
• dictmember
• enum
• const

And for HTML definitions:

• html-element
• html-attribute

And finally, a catch-all category for general terms and phrases, and anything that doesn't fall into one of the above categories:

• dfn

The processor will attempt to infer your definition type from the context and text content of the definition:

• Is it inside a propdef or descdef block? Then it's a property or descriptor.
• Is it inside an idl block (<pre class='idl'>)? Then it's an interface.
• Does it start with an @? Then it's an at-rule.
• Is it surrounded by <>? Then it's a type.
• Is it surrounded by 〈〉? Then it's a token.
• Does it start with a :? Then it's a selector.
• Does it end with ()? Then it's a function.
• Is it surrounded by double single quotes in the source, like ''foo''? Then it's a value.
• Otherwise, it's a dfn.

Note that this auto-detection is a last-resort operation. There are methods (defined below) to explicitly indicate what type a definition is, and those win over the auto-detection.

If your value doesn't fit one of these categories, you'll have to tag it manually. Just add the type as a boolean attribute to the definition, like

  attribute DOMString <dfn attribute>name</dfn>;

Alternately, if you've got several definitions of the same type that share some container element (such as a <pre> or <dl>), just add a dfn-for="type-goes-here" attribute to the container. Anything which isn't explicitly tagged otherwise will take that type by default.

(There are more methods to determine definition type, but they're only meant for legacy content, and so are not documented here.)

Specifying What a Definition is For

Some types of definitions are defined relative to a higher construct, such as values for a particularly property, or attributes of a particular IDL interface. This is useful, as it means these names don't have to be globally unique, but that means your autolinks may have a hard time telling which name you intend to link to.

To fix this, the processor enforces that some types of definitions must define what they are for. This is specified with a for='' attribute on the definition.

Specifically:

• "attribute", "method", and "const" definitions must define what interface they're relative to.
• "dictmember" definitions must define what dictionary they're relative to.
• "descriptor" definitions must define what at-rule they're relative to. (This happens automatically if you add a "For" line to the descdef table.)
• "value" definitions must define what property, descriptor, at-rule, type, selector, or function they're relative to. If a value definition is relative to a descriptor, the value must be of the form "@foo/bar", where "@foo" is the at-rule the "bar" descriptor is relative to.

Just like with the definition type, you can instead declare what several definitions are for by putting an attribute on a container. In this case, just add dfn-for='' to the container. This is especially useful for property/descriptor values, as they're usually defined in a <dl>, or IDL definitions, as you can just put a dfn-for on the <pre class='idl'>.

Exporting Definitions

Most definitions are automatically "exported", which means they're made available for other specs to autolink to. The only exception is "dfn" type definitions, which aren't exported by default.

To force a link to be exported, add an export boolean attribute to it. To force a link not to be exported, add a noexport boolean attribute instead. Like the other attributes, you can instead add this to a container to have it be a default for the definitions inside.

Autolinking

The processor supports "autolinks" for easy linking to terms without having to fiddle around with urls. Instead, just match up the text of the link to the text of the definition!

In its most basic form, autolinks are just <a> elements without href='' attributes. The processor takes this as a signal that it should attempt to automatically determine the link target. It compares the text content of the link to the text content of all the definitions in the page or in the cross-ref data, and if it finds a match, automatically sets the href appropriately to point at the relevant definition.

Like definitions, you can override the linking text by setting a title='' attribute. Unlike definitions, you can't separate multiple linking phrases by the bar "|" character, as that doesn't make sense for links.

Setting an empty title attribute turns off autolinking entirely, if for whatever reason you need to do so.

There are several additional shortcuts for writing an autolink:

• <i> elements are treated as autolinks as well, for legacy reasons.
• 'foo' (apostophes/straight quotes) is an autolink to a property or descriptor named "foo"
• ''foo'' (double apostrophes) is an autolink to any of the CSS definition types except property and descriptor
• <<foo>> is an autolink to a type/production named "<foo>"
• <<'foo'>> is an autolink to the the property or descriptor named "foo" (used in grammars, where you need <foo> for non-terminals)
• <<foo()>> is an autolink to the function named "foo" (same)
• <<@foo>> is an autolink to the at-rule named "@foo" (same)
• <<<foo>>> is an autolink to a token named "〈foo〉"
• [[foo]] is an autolink to a bibliography entry named "foo", and auto-generates an informative reference in the biblio section. Add a leading exclamation point to the value, like [[!foo]] for a normative reference.

Link Types

Links have the same types as definitions, with a few additional "union" types that are used by the shortcut forms. While you shouldn't specify them explicitly, they'll show up in error messages sometimes, so here's a list of them:

• "propdesc" - used by the 'foo' shorthand. A union of "property" and "descriptor".
• "functionish" - used by the ''foo'' shorthand for things that look like functions. A union of "function" and "method".
• "maybe" - used by the rest of the ''foo'' shorthand values. A union of "dfn" and all the CSS types except "property" and "descriptor". For legacy reasons, this link type has the additional magic that it doesn't flag an error if it can't find any matches, because it's also used to annotate inline CSS code fragments.

Additionally, there's an "idl" link type which is intended to be used by authors. It's a union of all the WebIDL types, and exists because the WebIDL types are somewhat verbose.

When you actually run the processor, you may get errors about there being too many possible references to choose from. The processor will continue to run anyway, but its default choice might be the wrong definition. There are three things you might have to do to fix these:

1. Specify the type explicitly, if the link isn't being processed as the correct type. Like definitions, this can be done by just adding the type as a boolean attribute on the link, or by adding a link-for='' attribute to a container.

2. If the link type corresponds to one of the definition types that needs for to be specified, you may need to specify for on the link as well to narrow down which definition you're referring to. For example, many properties define an "auto" value; to link to the "auto" value of the 'width' property in particular, specify <a value for=width>auto</a>. To refer to a value of a descriptor, you can be completely explicit and specify the at-rule as well, like <a value for='@counter-style/system'>numeric</a>, but you're allowed to omit the at-rule if there are no other properties or descriptors with the same name, like <a value for='system'>numeric</a>. This might trigger errors in the future if a conflicting property or definition gets added later, but it keeps your links shorter for now.

   Again, you can specify a link-for='' attribute on a container to default it for all the autolinks inside the container.

3. If multiple specs define the same property, you may need to declare which spec you're referring to. (The processor is smart enough to automatically figure out which one you probably want in many cases.) Just add a spec='' attribute with the spec's shortname to either the link or a container. This can also be specified in the spec's metadata with "Link Defaults", which applies document-wide.

As a final note, the autolinking algorithm will link differently based on whether the spec being processed is an "unofficial" or "official" draft. If "unofficial" (ED, UD, etc.), it'll prefer to link to other EDs, and will only link to TRs if no ED version of that spec exists. (If a definition only exists in the "official" draft but not the editor's draft, that almost certainly means it's been deleted since the official draft was last published, and thus shouldn't be linked to.) On the other hand, "official" (WD, CR, etc.) specs will preferentially link to other official specs. A future version of the processor will likely enforce the W3C's linking policy more strongly: preventing CRs from linking to EDs at all, preventing RECs from linking to anything below CR, etc.

If you need to override the processor's choice for which status to link to for a particular link, provide a status='' attribute containing either "ED" or "TR" on the link or a container.

Bibliography

Bibliographical references form a special class of autolinks. They're typically added only via the shorthands [[FOO]] for informative references and [[!FOO]] for normative references.

If, for whatever reason, you need to craft a bibliography link manually, add data-link-type=biblio and data-biblio-type=[normative | informative] attributes to the link.

Unlike regular autolinks, which link to <dfn> elements, biblio autolinks cause the spec to generate entries in its "References" section, and then link to that instead. The biblio database is also entirely separate from the normal definitions database, and can be found at https://www.w3.org/Style/Group/css3-src/biblio.ref (member-only link, unfortunately). A version of this file is included in the processor's repository, and the data doesn't change often, so it should be sufficient.