Box Tree API Level 1

A Collection of Interesting Ideas,

This version:
https://drafts.css-houdini.org/box-tree-api-1/
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Abstract

Layout as described by CSS produces boxes that control how content is displayed and positioned. This specification describes an API for accessing information about these boxes.

Status of this document

1. Introduction

The layout stage of CSS is responsible for generating the position and size of a document’s content. During this process, each DOM element produces potentially many boxes, each of which in turn produce potentially many fragments.

This specification describes an API that gives developers access to geometry, text and other information about boxes and fragments.

2. Boxes and Fragments

The [css-display-3] specification describes the relationship between elements, boxes, and fragments.

Elements and pseudo-elements generate zero or more fragments, each of which generates a fragment tree.

Fragments do not in general form a tree that maps cleanly back to the DOM. When an element generates multiple fragment trees, the element that generates a least common ancestor can be arbitrarily far up the DOM tree.

Assuming that layout places "foo bar" on the first line, and "baz" on the second, the following HTML produces six fragments in a single tree.
<style>
p::first-line { color: green; }

p::first-letter { color: red; }
</style>
<p>foo <i>bar baz</i></p>

The fragments are:

The italic element produces two fragments ("bar" and "baz"), each in its own tree. In this example, the paragraph element generates the common root for these fragments; however if (for example) the paragraph element were itself a descendant of a multi-column div then the common root may be further up the tree.

Boxes are not explicitly exposed by this API.

3. API

interface DeadFragmentInformation {
  readonly attribute Node node;
  readonly attribute double width;
  readonly attribute double height;
  readonly attribute double top;
  readonly attribute double left;
  readonly attribute boolean isOverflowed;
  readonly attribute sequence<DeadFragmentInformation>? children;
  readonly attribute DeadFragmentInformation? nextSibling;
  readonly attribute DeadFragmentInformation? previousSibling;
  readonly attribute DeadFragmentInformation? nextInBox;
  readonly attribute DeadFragmentInformation? previousInBox;
};

enum FragmentFilter {
  "direct-fragments-only",
  "fragment-hierarchy"
};

partial interface Element {
  Promise<DeadFragmentInformation> getFragmentInformation(FragmentFilter filter);
};

partial interface Document {
  void layoutNow();
};

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-DISPLAY-3]
Elika Etemad. CSS Display Module Level 3. URL: https://drafts.csswg.org/css-display/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

IDL Index

interface DeadFragmentInformation {
  readonly attribute Node node;
  readonly attribute double width;
  readonly attribute double height;
  readonly attribute double top;
  readonly attribute double left;
  readonly attribute boolean isOverflowed;
  readonly attribute sequence<DeadFragmentInformation>? children;
  readonly attribute DeadFragmentInformation? nextSibling;
  readonly attribute DeadFragmentInformation? previousSibling;
  readonly attribute DeadFragmentInformation? nextInBox;
  readonly attribute DeadFragmentInformation? previousInBox;
};

enum FragmentFilter {
  "direct-fragments-only",
  "fragment-hierarchy"
};

partial interface Element {
  Promise<DeadFragmentInformation> getFragmentInformation(FragmentFilter filter);
};

partial interface Document {
  void layoutNow();
};