CSS Basic User Interface Module Level 4

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.

[CSS3-UI] was later introduced to incorporates, extends, and supersedes these. This specification continues this work, and in turn replaces [CSS3-UI].

Note: At the time of writing, [CSS3-UI] is not completely finalized yet. To avoid accidental divergences and maintenance overhead, This specification is written as a delta specification over CSS-UI Level 3. Once the level 3 specification is final, its content will be integrated into this specification, which will then replace it. Until then, CSS-UI Level 4 only contains additions and extensions to level 3.

1.1. Purpose

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

• Extend the user interface features in CSS2.1 and [CSS3-UI]
• 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 In addition, it replaces and supersedes [CSS3-UI], which itself replaced and superseded the following: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

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4. Outline properties

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4.1. outline property

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4.2. outline-width property

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4.3. outline-style property

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4.4. outline-color property

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5. Resizing & Overflow

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5.1. resize property

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5.2. Overflow Ellipsis: the text-overflow property

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This specification introduces a new value to the text-overflow property: fade, as well as its functional notation variant fade().

fade( <length> | <percentage> )

Clip inline content that overflows its line box. Characters may be only partially rendered. In addition, the UA must apply a fade out effect near the edge of the line box, reaching complete transparency at the edge.

Do we need to define the way the fade out is calculated so that the fading is identical across browsers? It should probably be something like mask-image: linear-gradient(to right, rgba(0,0,0,1), rgba(0,0,0,0)), except applied to the relevant portion of the line only.

The argument determines the distance over which the fade effect is applied. The <percentage> is resolved against the width of the line box. Values lower than 0 are clipped to 0. Values greater than the width of the line box are clipped to the width of the line box.

If the line box is too short to display the fade effect at the desired length, should we drop the effect, or shrink the distance it is applied over until it fits, or clip the end of the fade?

fade

Same as fade(), but the distance over which the fading effect is applied is determined by the UA. 1em is suggested as a reasonable value.

How should we deal with things overflowing out of the line box, or overlapping onto it? Should fade apply to the logical content of the line, or to the physical area of the line box, or the intersection of both?

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

6. Pointing Devices and Keyboards

6.1. Pointer interaction

6.1.1. cursor property

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Amend the definition of auto to show default rather than text over text when user-select is none.

6.2. Insertion caret

6.2.1. Coloring the insertion caret: caret-color

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Amend the definition of auto to highlight the fact that when caret-shape is block, ensuring good visibility and contrast means not using currentColor.

6.2.2. Animation of the insertion caret: caret-animation

On most platforms and in most UAs, the text insertion caret blinks. This property allows the author to control the speed at which it blinks, or to turn off blinking entirely.

auto

The UA determines how the caret should be animated, if at all. It should match platform conventions, and may be adjusted based on context.

Note: This is typically rendered as a blinking caret.

blink

The UA must display a blinking caret. For accessibility reasons, and based on user preferences, The UA may display a non animated caret instead.

none

The UA must not animate the caret.

Note: This is only about UA driven animations of the caret. If CSS animations are used to animate the caret color, these should still apply normally.

fade

The UA must display a caret repeatedly fading in and out, similarly to blink except it appears and disappears progressively rather than suddenly. For accessibility reasons, and based on user preferences, The UA may display a non animated caret instead.

Do we need the accessibility escape hatch on blink and fade? Would it be enough instead to expect the UI to put caret-animation:fixed !important in the user stylesheet when it wants to prevent the caret from blinking?

The UA determines the speed at which the cursor blinks or fades. It should follow platform conventions and settings.

Note: It is recommended to stop the caret from blinking or fading using caret-animation: none when applying custom animations using [CSS3-ANIMATIONS], to prevent the blinking or fading and the css animation to interfere.

Note: See Guideline 2.3: Seizures: Do not design content in a way that is known to cause seizures [WCAG20].

/* prevent the caret from blinking/flashing */
:read-write { caret-animation: none !important; }
/* prevent changes of caret-color, including animations */
:read-write { caret-color:auto !important; }

UAs that do not have an editable user stylesheet should provide a setting to disable blinking, flashing and animated carets. UAs that do have an editable user stylesheet may want to provide this setting as well. See [WCAG] Guideline 2.2 and Guideline 2.3 for details.

textarea {
        caret-animation: none;
        caret-color: blue;
        animation: caret-alternate 2s step-end infinite;
}
@keyframes caret-alternate {
  50% { caret-color: red; }
}

6.2.3. Shape of the insertion caret: caret-shape

This property allows authors to specify the desired shape of the text insertion caret.

Within the context of this definition, character is to be understood as extended grapheme cluster, as defined in [UAX29], and visible character means a character with a non-zero advance measure.

auto

The UA determines the shape of the caret. It should match platform conventions, and may be adjusted based on context. For example, if a UA switches between insert mode and overtype mode when the user presses the insert key on their keyboard, it may show a bar caret in insert mode, and a block caret in overtype mode.

bar

The UA must render the text insertion caret as a thin bar placed at the insertion point. This means it is between, before, or after characters, not over them. It should be perpendicular to the inline progression direction, although UAs may render it slanted when inserting italic or oblique text.

block

The UA must render the text insertion caret as a rectangle overlapping the next visible character following the insertion point. If there is no visible character after the insertion point, the UA must render the caret after the last visible character. UAs may render it as a slanted rectangle when inserting italic or oblique text.

underscore

The UA must render the text insertion caret as a thin line under (as defined in [CSS-WRITING-MODES-3] the next visible character following the insertion point. If there is no visible character after the insertion point, the UA must render the caret after the last visible character.

The width of the block and underscore carets should be the advance measure of the next visible character after the insertion point, or 1ch if there is no next visible character or if this information is impractical to determine.

When determining the orientation and appearance of the caret, UAs must take into account the writing mode [CSS-WRITING-MODES-3] and must apply transformations [CSS-TRANSFORMS-1]. If the edited text is laid out out on a path, for instance by using the SVG textPath element, UAs should also account for this.

The stacking position of the caret is left undefined, within the following constraints:

• The caret must not be obscured by the background of the element
• UAs must render block carets so that the character it overlaps with is not obscured by the caret

.console {
  caret-shape: underscore;
  background: black;
  color: white;
  font-family: monospace;
  padding: 1ex;
}

user@host:css-ui-4 $ ls -a
. .. Overview.bs Overview.html
user@host:css-ui-4 $  

user@host:css-ui-4 $ ls -a
. .. Overview.bs Overview.html
user@host:css-ui-4 $ 

6.2.4. Insertion caret shorthand: caret

This property is a shorthand for setting caret-color, caret-animation and caret-shape in one declaration. Omitted values are set to their initial values.

#old-screen {
  caret: block 0s;
  animation: old-caret 2s infinite;
  /*styling of the screen omitted for brevity */
}
@keyframes old-caret {
  from, 50% { caret-color: green; }
  75%, to { caret-color: transparent; }
}

6.3. Keyboard control

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

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6.3.2. Obsolete: the ime-mode property

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7. User Interaction

7.1. Controlling content selection

The user-select property enables authors to specify which elements in the document can be selected by the user and how. This allows for easier interactions when not all elements are equally useful to select, avoiding accidental selections of neighbouring content.

User Agents must not apply the user-select property to the ::first-line and ::first-letter pseudo elements.

Whether user-select should apply to ::before and ::after is an open question.

The computed value is the specified value, except:

1. on editable elements where the computed value is always contain regardless of the specified value
2. when the specified value is auto, which computes one of the other values as defined below

For the purpose of this specification, an editable element is either an editing host or a mutable form control with textual content, such as textarea.

Should there be constraints on what happens to the computed value on elements that are editable descendants of editing hosts? The semantics are not obvious. Maybe none should compute to text, or maybe all values should compute to text.

auto

The computed value of auto is determined as follows:

• If the element is an editable element, the computed value is contain
• Otherwise, if the computed value of user-select on the parent of this element is all, the computed value is all
• Otherwise, if the computed value of user-select on the parent of this element is none, the computed value is none
• Otherwise, the computed value is text

Note: This unusual combination of a non inherited property with an initial value of auto whose computed value depends on the parent element makes it possible to create what is effectively selective inheritance. This was initially proposed by Microsoft in IE to introduce a behavior similar to inheritance except that the contain value does not inherit.

text

The element imposes no constraint on the selection.

none

The UA must not allow selections to be started in this element.

A selection started outside of this element must not end in this element. If the user attempts to create such a selection, the UA must instead end the selection range at the element boundary.

Note: As of the time of writing, experimental implementations do not all behave like this. Firefox does. Chrome and Safari almost do: for a selection started after the element and trying to go backwards into the element they behave as specified here, but for a selection started before the element and trying to go into the element they behave as if the element has all and select it entirely. IE does not restrict selections started outside of the element from going into it at all. Another difference is that in Chrome and Safari, if the user attempts to start a selection inside a user-select: none, and to end the selection out of it, a selection will be created from the boundary of the element to the user-designated end-point. Firefox and Internet explorer behave as prescribed in this specification and do not create a selection at all.

However, if this element has descendants on which the computed value of user-select is not none, selections that start and end within these descendants are allowed.

The UA must allow selections to extend across this element, and must exclude this element from such a selection. An exception is made for UAs which do not support multiple ranges per selection, and they may include this element. If the element has descendants on which user-select does not compute to none, these descendants must be included in a selection extending across the element.

As user-select is a UI convenience mechanism, not a copy protection mechanism, the UA may provide an alternative way for the user to explicitly select the text even when user-select is none.

Note: none is not a copy protection mechanism, and using it as such is ineffective: User Agents are allowed to provide ways to bypass it, it will have no effect on legacy User Agents that do not support it, and the user can disable it through the user style sheet or equivalent mechanisms on UAs that do anyway. Instead, none is meant to make it easier for the user to select the content they want, by letting the author disable selection on UI elements that are not useful to select. Tools such as CSS validators, linters or in-browser developer tools are encouraged to use heuristics to detect and warn against incorrect or abusive usage that would hamper usability or violate common user expectations.

contain

UAs must not allow a selection which is started in this element to be extended outside of this element.

A selection started outside of this element must not end in this element. If the user attempts to create such a selection, the UA must instead end the selection range at the element boundary.

The UA must allow selections to extend across this element, and such selections must include the content of the element.

Note: At the time of writing, experimental implementations behave differently from eachother about selections started outside and selections going into the element. The behavior can be observed even on browsers that do not explicitly support contain by trying to select into a textarea or a contenteditable element.

Note: This was initially introduced as an experimental feature in Internet Explorer, under the name user-select: element.

all

The content of the element must be selected atomically: If a selection would contain part of the element, then the selection must contain the entire element including all its descendants. If the element is selected and the computed value of user-select on its parent is all, then the parent must be included in the selection, recursively.

If this element has descendants on which the computed value of user-select is not all and if a selection is entirely contained in these descendants, then the selection is not extended to include this whole element.

Note: Selections can include more than just text and extend over images, tables, videos, etc. The behavior when copying and pasting a such selections is out of scope for this specification.

The following additions are made to the UA stylesheet for HTML:

button, meter, progress, select { user-select: none; }

the list above is incomplete, and needs to include at least the various button-like variants of input.

8. Form Control Styling

8.1. Switching appearance

While the way most elements in a document look can be fully controlled by CSS, form controls are typically rendered by UAs using native UI controls of the host operating system, which can neither be replicated nor styled using CSS.

This specification introduces the appearance property to provide some control over this behavior. Using appearance: none allows authors to suppress the native style of form controls, so that CSS can be used to fully restyle them.

Should auto compute to none on regular elements? I would say no: If we did that, to be consistent, every time we introduced a new value, we would change what auto computes to on some elements, which doesn’t sounds desirable.

Note: This specification intentionally refrains from making the appearance of all possible form controls and sub-controls available as values, as had previously been attempted by earlier proposals for this property and by several UA vendors in experimental implementations. Experience has shown that such a list would be very long and not practical to maintain, and UAs would need to add non-standard values to account for the behavior of non-standard pseudo-elements sometimes used to implement form controls. Moreover, many values of such an enumeration only make sense on a single element or pseudo-element, and are never used outside of the UA stylesheet. Instead, this specification only provides auto, none. UAs cannot therefore use the appearance property in the UA stylesheet to give each control is native look and feel, and must use appearance:auto instead.

IE supports -webkit-appearance, and also includes the textfield and button values. Presumably this was done due to compatibility problems, so we may need to include it as well.

auto

UAs may render form controls using native controls of the host operating system or with a look and feel not otherwise expressible in CSS.

Elements other than form controls must be rendered as if none had been specified.

none

The element is rendered following the usual rules of CSS. Replaced elements other than form controls are not affected by this, and remain replaced elements. From controls are not made to look like native controls of the host operating system. See below for details.

Shouldn’t this be called "normal" instead? none makes it sound like the targeted element will disappear.

On form control elements where the computed value is auto UAs may disregard some CSS properties to ensure that the intended appearance is preserved, or because these property may not be meaningful for the chosen appearance. However, the following properties must not be disregarded:

• appearance
• display
• visibility
• position
• top
• right
• bottom
• left
• float
• clear

Are there more properties should include in this list? Should we remove some? Should whitelist the properties that are ok to ignore instead of blacklisting those that are not? Either way, UAs do ignore some properties when rendering form controls, so this specification needs to say something about this.

All decorative visual aspects of a form control which are not caused by a CSS style rule must be suppressed when the appearance is changed using appearance, even if the UA’s internal representation for this element was composed of multiple elements or pseudo elements combined together. For example, select is often rendered with an arrow on the right side indicating that the list will be expanded if the element is clicked, If the computed value of appearance is none, this must disappear.

However, the UA must preserve aspects of the form control which are necessary to operate the control with its original semantics. This does not include aspects of a control which are merely needed to observe the state the control is in, only those that are needed for the user to be able to modify the state of the control. The UA may however give them a different look and feel as long as it remains possible to operate the control. For example, the slider of an <input type=range> is preserved (or replaced by an equivalent mechanism) even if appearance is none as it would otherwise not be possible to change the value of the range with the mouse or touchscreen. On the other hand, the check mark in an <input type=checkbox checked> must be suppressed, as it merely indicates the state the element is in, which can be styled in different ways using the :checked selector.

UAs are inconsistent about the preceding two paragraphs. What is specified here attempts to give some logic to what is preserved and what is not, based on the use-cases for appearance.

UAs should include in their User Agent stylesheet style rules to give form controls a recognizable shape when appearance is none.

Note: Authors may therefore need to override these UA style rules to get the styling they intended, possibly by using all: unset.

This usage of the all property would remove focus indicators created by the outline property, which seems undesirable. Using all: revert would not solve it, as it would fail to remove the UA styles as intended. How can we mitigate this?

The behavior and semantics of elements remain defined by the host language, and are not influenced by this property. For example, regardless of the computed value of appearance:

• Elements which can be in different states keep this ability, and the relevant pseudo-classes match as usual.
• If a select element is activated, the UI to choose one of the associated option elements is shown (although it may look different).
• Event handlers attached to the element trigger as usual.

Conversely, changing the appearance of an element must not cause it to acquire new semantics, pseudo classes, event handlers, etc that it did not initially have. The ability for an element to be focused is also not changed by the appearance property.

input[type=checkbox] {
  appearance: none;
  all: unset;
  width: 1em;
  height: 1em;
  display: inline-block;
  background: red;
}
input[type=checkbox]:checked {
  border-radius: 50%;
  background: green;
}

Appendix A. Acknowledgments

This appendix is informative.

This specification builds upon [CSS3-UI], which 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.

Later work on level 3 and on this level 4 was done by Florian Rivoal, working on behalf of Bloomberg.

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.

This appendix describes functional changes from the Level 3 Candidate Recommendation (CR) of 7 Jul 2015.

• The user-select property has been added
• The appearance property has been added
• The fade and fade() values have been added to text-overflow
• The caret shorthand property and the caret-shape and caret-animation properties have been added.

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?

   No.

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-* 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.

    The appearance property also allows author to turn off native styling and replace it with css based styling.

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

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