CSS Basic User Interface Module Level 3 (CSS3 UI)

Copyright © 2015 W3C^® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and document use rules apply.

Abstract

Status of this document

Table of Contents

1. Introduction

This module describes CSS properties which enable authors to style user interface related properties and values.

Section 2.1 of CSS1 [CSS1] and Chapter 18 of CSS2 [CSS2] introduced several user interface related properties and values. User Interface for CSS3 (16 February 2000) introduced several new user interface related features.

This Working Draft incorporates, extends, and supersedes them.

1.1. Purpose

The purpose of this specification is to achieve the following objectives:

• Extend the user interface features in CSS2.1.
• Provide additional CSS mechanisms to augment or replace other dynamic presentation related features in HTML.
• Introduce directional navigation properties to assist in the construction of user interfaces which make use of a directional navigation model.

2. Module Interactions

This document defines new features not present in earlier specifications. In addition, it replaces and supersedes the following:

• Section 18.1, section 18.4, and Information on the stacking of outlines defined in Appendix E of Cascading Style Sheets, level 2, revision 1 [CSS21]
• User Interface for CSS3 (16 February 2000) [CSSUI]

3. Box Model addition

3.1. box-sizing property

Note: This is the behavior of width and height as commonly implemented by legacy HTML user agents for replaced elements and input elements.

Note: In contrast to the length and percentage values, the auto value of the width and height properties (as well as other keyword values introduced by later specifications, unless otherwise specified) is not influenced by the box-sizing property, and always sets the size of the content box.

The following terms, whose definitions vary based on the computed value of box-sizing are introduced:

The Visual formatting model details of [CSS21] are written assuming box-sizing: content-box. The following disambiguations are made to clarify the behavior for all values of box-sizing:

1. In 10.3.3, the second width in the following phrase is to be interpreted as content width: If width is not auto and border-left-width + padding-left + width + [...]
2. In 10.3.7, width is to be interpreted as content width in the following equation: left + margin-left + border-left-width + padding-left + width + [...]
3. In 10.4, width, height, min-width, max-width, min-height and max-height are respectively to be interpreted as content width, content height, min inner width, max inner width, min inner height and max inner height in the following phrases:
   1. The tentative used width is calculated [...]
   2. If the tentative used width is greater than max-width, the rules above are applied again, but this time using the computed value of max-width as the computed value for width.
   3. If the resulting width is smaller than min-width, the rules above are applied again, but this time using the value of min-width as the computed value for width.
   4. Select from the table the resolved height and width values for the appropriate constraint violation. Take the max-width and max-height as max(min, max) so that min ≤ max holds true. In this table w and h stand for the results of the width and height computations [...]
   5. All instances of these words in the table
   6. Then apply the rules under "Calculating widths and margins" above, as if width were computed as this value.
4. In 10.6.4, height is to be interpreted as content height in the following equation: top + margin-top + border-top-width + padding-top + height + [...]
5. In 10.7, width, height, min-height and max-height are respectively to be interpreted as content width, content height, min inner height and max inner height in the following phrases:
   1. The tentative used height is calculated [...]
   2. If this tentative height is greater than max-height, the rules above are applied again, but this time using the value of max-height as the computed value for height.
   3. If the resulting height is smaller than min-height, the rules above are applied again, but this time using the value of min-height as the computed value for height.
   4. [...] use the algorithm under Minimum and maximum widths above to find the used width and height. Then apply the rules under "Computing heights and margins" above, using the resulting width and height as if they were the computed values.

div.container {
  width:38em;
  border:1em solid black;
}

div.split {
  box-sizing:border-box;
  width:50%;
  border:1em silver ridge;
  float:left;
}

<div class="container">
<div class="split">This div occupies the left half.</div>
<div class="split">This div occupies the right half.</div>
</div>

4. Outline properties

At times, style sheet authors may want to create outlines around visual objects such as buttons, active form fields, image maps, etc., to make them stand out. Outlines differ from borders in the following ways:

1. Outlines do not take up space.
2. Outlines may be non-rectangular.
3. UAs often render outlines on elements in the :focus state.

The outline properties control the style of these dynamic outlines.

The stacking of the rendering of these outlines is explicitly left up to implementations to provide a better user experience per platform. This supersedes the stacking of outlines as defined in CSS 2.1.

Keyboard users, in particular people with disabilities who may not be able to interact with the page in any other fashion, depend on the outline being visible on elements in the :focus state, thus authors must not make the outline invisible on such elements without making sure an alternative highlighting mechanism is provided.

The rendering of applying transforms to outlines is left explicitly undefined in CSS3-UI.

4.1. outline property

4.2. outline-width property

4.3. outline-style property

4.4. outline-color property

The outline created with the outline properties is drawn "over" a box, i.e., the outline is always on top, and doesn’t influence the position or size of the box, or of any other boxes. Therefore, displaying or suppressing outlines does not cause reflow.

Outlines may be non-rectangular. For example, if the element is broken across several lines, the outline should be an outline or minimum set of outlines that encloses all the element’s boxes.

Each part of the outline should be fully connected rather than open on some sides (as borders on inline elements are when lines are broken).

The parts of the outline are not required to be rectangular. To the extent that the outline follows the border edge, it should follow the border-radius curve.

The position of the outline may be affected by descendant boxes.

User agents should use an algorithm for determining the outline that encloses a region appropriate for conveying the concept of focus to the user.

Note: This specification does not define the exact position or shape of the outline, but it is typically drawn immediately outside the border box.

The outline-width property accepts the same values as border-width ([CSS3BG], Section 4.3).

The outline-style property accepts the same values as border-style ([CSS3BG], Section 4.2), except that hidden is not a legal outline style. In addition, in CSS3, outline-style accepts the value auto. The auto value permits the user agent to render a custom outline style, typically a style which is either a user interface default for the platform, or perhaps a style that is richer than can be described in detail in CSS, e.g. a rounded edge outline with semi-translucent outer pixels that appears to glow. As such, this specification does not define how the outline-color is incorporated or used (if at all) when rendering auto style outlines. User agents may treat auto as solid.

The outline-color property accepts all colors, as well as the keyword invert. Invert is expected to perform a color inversion on the pixels on the screen. This is a common trick to ensure the focus border is visible, regardless of color background.

Conformant UAs may ignore the invert value on platforms that do not support color inversion of the pixels on the screen.

If the UA does not support the invert value then it must reject that value at parse-time, and the initial value of the outline-color property is the currentColor [CSS3COLOR] keyword.

The outline property is a shorthand property, and sets all three of outline-style, outline-width, and outline-color.

Note: The outline is the same on all sides. In contrast to borders, there are no outline-top or outline-left etc. properties.

This specification does not define how multiple overlapping outlines are drawn, or how outlines are drawn for boxes that are partially obscured behind other elements.

button { outline: thick solid }

Graphical user interfaces may use outlines around elements to tell the user which element on the page has the focus. These outlines are in addition to any borders, and switching outlines on and off should not cause the document to reflow. The focus is the subject of user interaction in a document (e.g., for entering text, selecting a button, etc.).

:focus  { outline: thick solid black }
:active { outline: thick solid red }

Note: Since the outline does not affect formatting (i.e., no space is left for it in the box model), it may well overlap other elements on the page.

4.5. outline-offset property

By default, the outline is drawn starting just outside the border edge. However, it is possible to offset the outline and draw it beyond the border edge.

If the computed value of outline-offset is anything other than 0, then the outline is outset from the border edge by that amount.

:focus,:active  { outline-offset: 2px }

Negative values must cause the outline to shrink into the border box. Both the height and the width of outside of the shape drawn by the outline must not become smaller than twice the computed value of the outline-width property, to make sure that an outline can be rendered even with large negative values. User Agents must apply this constraint independently in each dimension. If the outline is drawn as multiple disconnected shapes, this constraint applies to each shape separately.

Note: Negative value constraints are an at risk feature due to interoperability problems, and thus might be dropped from a must to a should.

5. Resizing & Overflow

CSS2.1 provides a mechanism for controlling the appearance of a scrolling mechanism (e.g. scrollbars) on block container elements. This specification adds to that a mechanism for controlling user resizability of elements as well as the ability to specify text overflow behavior.

5.1. resize property

The resize property allows the author to specify whether or not an element is resizable by the user, and if so, along which axis/axes.

Currently it is possible to control the appearance of the scrolling mechanism (if any) on an element using the overflow property (e.g. overflow: scroll vs. overflow: hidden etc.). The purpose of the resize property is to also allow control over the appearance and function of the resizing mechanism (e.g. a resize box or widget) on the element.

Note: The resizing mechanism is NOT the same as the scrolling mechanism. The scrolling mechanism allows the user to determine which portion of the contents of an element is shown. The resizing mechanism allows the user to determine the size of the element.

The resize property applies to elements whose computed overflow value is something other than visible.

The effect of the resize property on generated content is undefined. Implementations should not apply the resize property to generated content.

Note: the resize property may apply to generated content in the future if there is implementation of Interface CSSPseudoElement.

When an element is resized by the user, the user agent sets the width and height properties to px unit length values of the size indicated by the user, in the element’s style attribute DOM, replacing existing property declaration(s) if any, without !important if any.

If an element is resized in only one dimension, only the corresponding property is set, not both.

The precise direction of resizing (i.e. altering the top left of the element or altering the bottom right) may depend on a number of CSS layout factors including whether the element is absolutely positioned, whether it is positioned using the right and bottom properties, whether the language of the element is right-to-left etc. The UA should consider the direction of resizing (as determined by CSS layout), as well as platform conventions and constraints when deciding how to convey the resizing mechanism to the user.

The user agent must allow the user to resize the element with no other constraints than what is imposed by min-width, max-width, min-height, and max-height. (The "must" is at risk since only Firefox currently supports this, and may be downgraded to a "should").

Note: There may be situations where user attempts to resize an element to appear to be overriden or ignored, e.g. because of !important cascading declarations that supersede that element’s style attribute width and height properties in the DOM.

Changes to the computed value of an element’s resize property do not reset changes to the style attribute made due to user resizing of that element.

iframe,object[type^="text/"],
object[type$="+xml"],object[type="application/xml"]
{
  overflow:auto;
  resize:both;
}

5.2. Overflow Ellipsis: the text-overflow property

This property specifies rendering when inline content overflows its line box edge in the inline progression direction of its block container element ("the block") that has overflow other than visible.

Text can overflow for example when it is prevented from wrapping (e.g. due to white-space: nowrap or a single word is too long to fit). Values have the following meanings:

Note: The <string> value, and the 2-value syntax "{1,2}" and functionality are all at risk.

The term "character" is used in this property definition for better readability and means "grapheme cluster" [UAX29] for implementation purposes.

If there is one value, it applies only to the end line box edge. If there are two values, the first value applies to the line-left edge, and the second value applies to the line-right edge. The terms end, line-left and line-right are defined in [CSS3-WRITING-MODES].

Note: the use of line-left and line-right rather than start and end when there are two values is intentional, to facilitate the use of directional characters such as arrows.

For the ellipsis and string values, implementations must hide characters and atomic inline-level elements at the applicable edge(s) of the line as necessary to fit the ellipsis/string. Place the ellipsis/string immediately adjacent to the applicable edge(s) of the remaining inline content. The first character or atomic inline-level element on a line must be clipped rather than ellipsed.

ellipsing details

• Ellipsing only affects rendering and must not affect layout nor dispatching of pointer events.
• The ellipsis is styled and baseline-aligned according to the block.
• Ellipsing occurs after relative positioning and other graphical transformations.
• If there is insufficient space for the ellipsis, then clip the rendering of the ellipsis itself (on the same side that neutral characters on the line would have otherwise been clipped with the text-overflow:clip value).

user interaction with ellipsis

• When the user is interacting with content (e.g. editing, selecting, scrolling), the user agent may treat text-overflow ellipsis or string values as text-overflow:clip.
• Selecting the ellipsis should select the ellipsed text. If all of the ellipsed text is selected, UAs should show selection of the ellipsis. Behavior of partially-selected ellipsed text is up to the UA.

Example(s):

text-overflow examples

These examples demonstrate setting the text-overflow of a block container element that has text which overflows its dimensions:

sample CSS for a div:

div {  font-family:Helvetica,sans-serif; line-height:1.1;
  width:3.1em; padding:.2em; border:solid .1em black; margin:1em 0;
}

sample HTML fragments, renderings, and your browser:

<div>
CSS IS AWESOME, YES
</div>

<div style="text-overflow:clip; overflow:hidden">
CSS IS AWESOME, YES
</div>

<div style="text-overflow:ellipsis; overflow:hidden">
CSS IS AWESOME, YES
</div>

<div style="text-overflow:ellipsis; overflow:hidden">
NESTED
 <p>PARAGRAPH</p>
WON’T ELLIPSE.
</div>

Note: the side of the line that the ellipsis is placed depends on the direction of the block. E.g. an overflow hidden right-to-left (direction: rtl) block clips inline content on the left side, thus would place a text-overflow ellipsis on the left to represent that clipped content.

ellipsis interaction with scrolling interfaces

This section applies to elements with text-overflow other than text-overflow:clip (non-clip text-overflow) and overflow:scroll.

When an element with non-clip text-overflow has overflow of scroll in the inline progression dimension of the text, and the browser provides a mechanism for scrolling (e.g. a scrollbar on the element, or a touch interface to swipe-scroll, etc.), there are additional implementation details that provide a better user experience:

When an element is scrolled (e.g. by the user, DOM manipulation), more of the element’s content is shown. The value of text-overflow should not affect whether more of the element’s content is shown or not. If a non-clip text-overflow is set, then as more content is scrolled into view, implementations should show whatever additional content fits, only truncating content which would otherwise be clipped (or is necessary to make room for the ellipsis/string), until the element is scrolled far enough to display the edge of the content at which point that content should be displayed rather than an ellipsis/string.

div.crawlbar {
  text-overflow: ellipsis;
  height: 2em;
  overflow: scroll;
  white-space: nowrap;
  width: 15em;
  border:1em solid black;
}

<div class="crawlbar">
CSS is awesome, especially when you can scroll
to see extra text instead of just
having it overlap other text by default.
</div>

As some content is scrolled into view, it is likely that other content may scroll out of view on the other side. If that content’s block container element is the same that’s doing the scrolling, and the computed value of text-overflow has two values, with the value applying to the start edge being a non-clip value, then implementations must render an ellipsis/string in place of the clipped content, with the same details as described in the value definition above, except that the ellipsis/string is drawn in the start (rather than end) of the block’s direction (per the direction property).

While the content is being scrolled, implementations may adjust their rendering of ellipses/strings (e.g. align to the box edges rather than line edges).

If there is insufficient space for both start and end ellipses/strings, then only the end ellipsis/string should be rendered.

6. Pointing Devices and Keyboards

6.1. Pointer interaction

6.1.1. cursor property

This property specifies the type of cursor to be displayed for the pointing device when within the element’s border edge.

Note: As per [CSS3-BACKGROUND], the border edge is affected by border-radius.

User agents may ignore the cursor property over native user-agent controls such as scrollbars, resizers, or other native UI widgets e.g. those that may be used inside some user agent specific implementations of form elements.

Values have the following meanings:

The UA must support the following image file formats:

• PNG, as defined in [PNG]
• SVG, as defined in [SVG], in secure static mode [SVG-INTEGRATION]
• any other non-animated image file format that they support for <image> in other properties, such as the the background-image property

In addition, the UA should support the following image file formats:

• SVG, as defined in [SVG], in secure animated mode [SVG-INTEGRATION]
• any other animated image file format that they support for <image> in other properties, such as the the background-image property

The UA may also support additional file formats.

Note: At the time of writing this specification (spring 2015), the only file formats supported for cursors in common desktop browsers are the .ico and .cur file formats, as designed by Microsoft. For compatibility with legacy content, UAs are encouraged to support these, even though the lack of an open specification makes it impossible to have a normative requirement about these formats. Some information on these formats can be found on Wikipedia.

The default object size for cursor images is a UA-defined size that should be based on the size of a typical cursor on the UA’s operating system.

The concrete object size is determined using the default sizing algorithm. If an operating system is incapable of rendering a cursor above a given size, cursors larger than that size must be shrunk to within the OS-supported size bounds, while maintaining the cursor image’s intrinsic ratio, if any.

The optional <x> and <y> coordinates identify the exact position within the image which is the pointer position (i.e., the hotspot).

Note: This specification does not define how the coordinate systems of the various types of <image> are established, and defers these definitions to [CSS4-IMAGES].

If the values are unspecified, then the intrinsic hotspot defined inside the image resource itself is used. If both the values are unspecific and the referenced cursor has no defined hotspot, the effect is as if a value of "0 0" were specified.

If the coordinates of the hotspot, as specified either inside the image resource or by <x> and <y> values, fall outside of the cursor image, they must be clamped (independently) to fit.

:link,:visited {
    cursor: url(example.svg#linkcursor),
            url(hyper.cur),
            url(hyper.png) 2 3,
            pointer
}

6.1.1.1. Cursor of the canvas

The document canvas is the infinite surface over which the document is rendered [CSS21]. Since no element corresponds to the canvas, in order to allow styling of the cursor when not over any element, the canvas cursor re-uses the root element’s cursor. However, if no boxes are generated for the root element (for example, if the root element has display: none), then the canvas cursor is the platform-dependent default cursor.

Note: An element might be invisible, but still generate boxes. For example, if the element has visibility: hidden but not display: none, boxes are generated for it and its cursor is used for the canvas.

6.2. Insertion caret

6.2.1. Coloring the insertion caret: caret-color

The caret is a visible indicator of the insertion point in an element where text (and potentially other content) is inserted by the user. This property controls the color of that visible indicator.

Note: caret shape and blinking is outside the scope of this feature and thus unspecified.

Note: The caret-color property is at risk.

6.3. Keyboard control

6.3.1. Directional focus navigation: the nav-up, nav-right, nav-down, nav-left properties

The <id> value is an ID selector [SELECT]. In response to directional navigation input corresponding to the property, the focus is navigated to the first element in tree order matching the selector.

If this refers to the currently focused element, the directional navigation input respective to the nav- property is ignored — there is no need to refocus the same element.

If no element matches the selector, the user agent automatically determines which element to navigate the focus to.

If the focus is navigated to an element that was not otherwise focusable, it becomes focusable only as the result of this directional navigation, and the :focus pseudo-class matches the element while it is focused as such.

Note: there were other options under consideration for such "not otherwise focusable" elements, including focus to the next otherwise focusable element in the document tree (including descendants of such a not otherwise focusable element). Input on such other options is welcome and explicitly solicited, especially from implementation experiences and author experience using the directional navigation properties in their content.

User agents for devices with directional navigation keys respond by navigating the focus according to four respective nav-* directional navigation properties (nav-up, nav-right, nav-down, nav-left). This specification does not define which keys of a device are directional navigational keys.

Note: Typical personal computers have keyboards with four arrow keys. One possible implementation would be to use those four arrow keys for directional navigation. For accessibility and user convenience, user agents should allow configuration of which keys on a keyboard are used for directional navigation.

button { position:absolute }

button#b1 {
  top:0; left:50%;
  nav-right:#b2; nav-left:#b4;
  nav-down:#b2; nav-up:#b4;
}

button#b2 {
  top:50%; left:100%;
  nav-right:#b3; nav-left:#b1;
  nav-down:#b3; nav-up:#b1;
}

button#b3 {
  top:100%; left:50%;
  nav-right:#b4; nav-left:#b2;
  nav-down:#b4; nav-up:#b2;
}

button#b4 {
  top:50%; left:0;
  nav-right:#b1; nav-left:#b3;
  nav-down:#b1; nav-up:#b3;
}

button#foo { nav-left: #bar "sidebar"; }

6.3.2. Obsolete: the ime-mode property

"ime-mode" is a property somewhat implemented in some browsers, that is problematic and officially obsoleted by this specification.

There is documentation of non-interoperability of these implementations.

User agents should not support the ime-mode property.

Authors must not use the ime-mode property.

Users may use the ime-mode property only for repair use-cases where they have to work around bad sites and legacy implementations, e.g. with a user style sheet rule like:

input[type=password] {    ime-mode: auto !important;
}

This example CSS may be placed into a user style sheet file to force password input fields to behave in a default manner.

This specification deliberately does not attempt to document the functionality of legacy ime-mode implementations nor what they specifically support because it does not make sense to pursue or recommend any such path.

Appendix A. Acknowledgments

This appendix is informative.

This specification was edited and written for the most part by Tantek Çelik from 1999 to the present, first while representing Microsoft, then as an invited expert, and most recently while representing Mozilla.

Thanks to Florian Rivoal, working on this specification on behalf of Bloomberg, for his recent work documenting issues from www-style emails, proposing resolutions & changes, and in particular for researching & writing greatly improved details for the box-sizing property.

Thanks to feedback and contributions from Rossen Atanassov, Tab Atkins, L. David Baron, Bert Bos, Matthew Brealey, Rick Byers, Ada Chan, James Craig, Michael Cooper, Axel Dahmen, Michael Day, Micah Dubinko, Elika E., Steve Falkenburg, Andrew Fedoniouk, Al Gilman, Ian Hickson, Bjoern Hoehrmann, Alan Hogan, David Hyatt, Richard Ishida, Sho Kuwamoto, Yves Lafon, Stuart Langridge, Susan Lesch, Peter Linss, Kang-Hao Lu, Masayuki Nakano, Mats Palmgren, Brad Pettit, François Remy, Andrey Rybka, Simon Sapin, Alexander Savenkov, Sebastian Schnitzenbaumer, Lea Verou, Etan Wexler, David Woolley, Frank Yan, Boris Zbarsky, and Domel.

Appendix B. Changes

This appendix is informative.

In general this draft contains editorial/grammatical/spelling corrections, and a few new informative examples. This appendix describes functional changes from the Working Draft (WD) of 17 Jan 2012 that were made to reflect implementer adoption (or lack thereof), as well as implementer innovations, and feedback from the general public. In particular, changes since the previous draft fall into one of three categories:

• Minor refinements to features based on implementation experience.
• Dropping features that were not implemented, or were insufficiently implemented to exit CR.
• Adding a small number of new but already interoperably implemented features.

List of substantial changes since the 2015-05-19 Working Draft

• Strengthened accessibility related messages about use and abuse of the outline property
• Changed the handling of unsupported cursor values to standard CSS error handling.
• Dropped the padding-box value of the box-sizing property due to no implementation interest from UA vendors.
• Clarified that the cursor property affects the shape of the cursor when within the border edge
• Added 'resize: both' to the default HTML stylesheet for textarea elements

List of substantial changes since the 2015-04-09 Working Draft

• box-sizing effects on CSS 2.1 width and height computations specified much more explicitly.
• cursor image formats normatively required.
• caret-color is inherited and animatable.
• New informative appendix on Considerations for Security and Privacy.

List of substantial changes since the 2015-03-10 Working Draft

Only editorial changes were made.

List of substantial changes since the 2015-02-24 Working Draft

• ::value and ::choices pseudo-elements have been dropped due to insufficient implementer interest. As they were published in a previous CR draft, they may be re-added in a future level if implementers show interest and intent to implement.

List of substantial changes since the 2012-02-17 Last Call Working Draft

• ::value pseudo-element. Added detail about which properties apply to the ::value pseudo-element.
• text-overflow property - added <string> value and 2 values option, both explicitly at risk.
• nav-up, nav-right, nav-down, and nav-left properties removed from the "at risk" list.
• Dropped the nav-index property due to lack of implementation interest.
• cursor values "grab" and "grabbing" added per multiple interoperable implementations.
• Numerous UI selectors moved to Selectors4 and CSS Pseudo module.
• add caret-color property, but at risk.
• ime-mode explicitly obsoleted.
• outline-offset define negative values.
• Dropped the icon property.
• Dropped the icon value of the content property.
• XForms-only pseudo-elements dropped.

Appendix C. Considerations for Security and Privacy

This appendix is informative.

The W3C TAG is developing a Self-Review Questionnaire: Security and Privacy for editors of specifications to informatively answer.

Per the Questions to Consider

1. Does this specification deal with personally-identifiable information?

   No.

2. Does this specification deal with high-value data?

   No.

3. Does this specification introduce new state for an origin that persists across browsing sessions?

   No.

4. Does this specification expose persistent, cross-origin state to the web?

   No.

5. Does this specification expose any other data to an origin that it doesn’t currently have access to?

   No.

6. Does this specification enable new script execution/loading mechanisms?

   Yes. The cursor property accepts <image> values which may include URLs to be loaded.

7. Does this specification allow an origin access to a user’s location?

   No.

8. Does this specification allow an origin access to sensors on a user’s device?

   Yes. The directional focus navigation properties indirectly allow access to the device’s keyboard navigation input mechanism such as arrow keys.

9. Does this specification allow an origin access to aspects of a user’s local computing environment?

   No.

10. Does this specification allow an origin access to other devices?

    No.

11. Does this specification allow an origin some measure of control over a user agent’s native UI?

    Yes. The cursor and caret-color properties enable the page to change the display of the cursor and text insertion caret of the user agent’s native UI. In addition the outline-style property’s auto value (and thus outline shorthand) enable the page to potentially display a native focused element outline presentation around any element.

12. Does this specification expose temporary identifiers to the web?

    No.

13. Does this specification distinguish between behavior in first-party and third-party contexts?

    No.

14. How should this specification work in the context of a user agent’s "incognito" mode?

    No differently.

15. Does this specification persist data to a user’s local device?

    No.

16. Does this specification have a "Security Considerations" and "Privacy Considerations" section?

    Yes.

17. Does this specification allow downgrading default security characteristics?

    No.

Appendix D. Default style sheet additions for HTML

This appendix is informative.

Potential additions to the base style sheet to express HTML form controls, and a few dynamic presentation attributes:

:enabled:focus {
 outline: 2px inset;
}

button,
input[type=button],
input[type=reset],
input[type=submit],
input[type=checkbox],
input[type=radio],
textarea,
input,
input[type=text],
input[type=hidden],
input[type=password],
input[type=image]
{
 display: inline-block;
 white-space: nowrap;
}

button
{
/* white space handling of BUTTON tags in particular */
 white-space:normal;
}

input[type=reset]:lang(en)
{
/* default content of HTML input type=reset button, per language */
 content: "Reset";
}

input[type=submit]:lang(en)
{
/* default content of HTML input type=submit button, per language */
 content: "Submit";
}

/* UAs should use language-specific Reset/Submit rules for others. */

input[type=button],
input[type=reset][value],
input[type=submit][value]
{
/* text content/labels of HTML "input" buttons */
 content: attr(value);
}

textarea
{
/* white space handling of TEXTAREA tags in particular */
 white-space:pre-wrap;
 resize: both;
}

input[type=hidden]
{
/* appearance of the HTML hidden text field in particular */
 display: none;
}

input[type=image]
{
 display: inline-block;
 content: attr(src,url);
 border: none;
}

select[size]
{
/* HTML4/XHTML1 <select> w/ size more than 1 - appearance of list */
 display: inline-block;
 height: attr(size,em);
}

select,select[size=1]
{
/* HTML4/XHTML1 <select> without size, or size=1 - popup-menu */
 display: inline-block;
 height: 1em;
 overflow: hidden;
}

select[size]:active
{
/* active HTML <select> w/ size more than 1 - appearance of active list */
 display: inline-block;
}

optgroup,option
{
 display: block;
 white-space: nowrap;
}

optgroup[label],option[label]
{
 content: attr(label);
}

option[selected]::before
{
 display: inline;
 content: check;
}

/* Though FRAME resizing is not directly addressed by this specification,
   the following rules may provide an approximation of reasonable behavior. */

/*

frame { resize:both }
frame[noresize] { resize:none }

*/

Appendix E: Test Suite

This appendix is informative.

There is a partial test suite written according to the CSS Test Suite Documentation and following the CSS2.1 Test Case Authoring Guidelines. The test suite shall allow user agents to verify their basic conformance to the specification. This test suite does not pretend to be exhaustive and does not cover all possible combinations of user interface related features. These tests will be made available from the CSS Test Suites home page.

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:

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:

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.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

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.

CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or "nightly build"). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Index

Terms defined by this specification

• border-box, 3.1
• box-sizing, 3.1
• caret-color, 6.2.1
• clip, 5.2
• content-box, 3.1
• cursor, 6.1.1
• ellipsis, 5.2
• invert, 4.4
• max inner height, 3.1
• max inner width, 3.1
• min inner height, 3.1
• min inner width, 3.1
• nav-down, 6.3.1
• nav-left, 6.3.1
• nav-right, 6.3.1
• nav-up, 6.3.1
• outline, 4.1
• outline-color, 4.4
• outline-offset, 4.5
• outline-style, 4.3
• outline-width, 4.2
• resize, 5.1
• text-overflow, 5.2

Terms defined by reference

• [CSS21] defines the following terms:
  • border edge
  • bottom
  • left
  • overflow
  • right
  • top
  • visible
• [css-backgrounds-3] defines the following terms:
  • background-image
  • border-bottom-width
  • border-left-width
  • border-radius
  • border-right-width
  • border-style
  • border-top-width
  • border-width
  • solid
• [css-color-3] defines the following terms:
  • <color>
  • color
• [css-images-3] defines the following terms:
  • <image>
  • concrete object size
  • default object size
  • default sizing algorithm
• [css-values-3] defines the following terms:
  • <length>
  • <number>
  • <string>
  • <url>
• [css-writing-modes-3] defines the following terms:
  • direction
  • end
  • left
  • line-left
  • line-right
  • start
• [CSS21] defines the following terms:
  • content height
  • content width
  • display
  • height
  • invert
  • margin-left
  • margin-top
  • max-height
  • max-width
  • min-height
  • min-width
  • padding-bottom
  • padding-left
  • padding-right
  • padding-top
  • visibility
  • width

References

Normative References

Informative References

Property Index