CSS Round Display Level 1

1. Introduction

Everything on the web is a rectangle. For example, the window content area in a web browser is a rectangle. Each HTML element follows the W3C box model and thus is also a rectangle. New devices with a round display are now emerging. The current web standards lack some features to support the devices as follows:

Lack of the capability to detect a round display
Lack of layout mechanisms suitable for a round display

In order to facilitate the use of the web on a round display, we need to rethink existing CSS features.

Current user agents are not capable of detecting the shape of a display so that authors cannot apply various layouts for a round display. To resolve the issue, we add the device-radius media feature to Media Queries. The feature informs the web page of the property regarding the shape of the display.

To apply the shape of a display to content area, we extend the shape-inside property of CSS Shapes. We also add the border-boundary property to CSS Borders and introduce polar positioning for a better web design suitable for a round display.

2. Terminology

This specification follows the CSS property definition conventions from [CSS21].
The detailed description of Media Queries is defined in [MEDIAQUERIES-4]
The detailed description of CSS Shapes is defined in [CSS-SHAPES-1]
The detailed description of Borders is defined in [CSS3-BORDER]
The detailed description of Positioned Layout is defined in [CSS3-POSITIONING]

3. Extending Media Queries for a round display

Media Queries [MEDIAQUERIES-4] define mechanisms to support media-dependent style sheets, tailored for different environments. We propose to extend Media Queries by adding the device-radius media feature to support a round display. This will allow web authors to apply different styles to a web page on the rounded display.

3.1. The device-radius media feature

To use different style sheets for a rectangle display and for a round display, media queries should support some properties to identify the display shape. The device-radius media feature describes the property of rounded borders of a display.

This media query indicates that different style sheets will be applied depending on the display shape.

<link media="screen and (device-radius: 0%)" rel="stylesheet" href="rectangle.css" />
<link media="screen and (device-radius: 50%)" rel="stylesheet" href="round.css" />

By the mechanism of media queries, if the value of the device-radius media feature is less than 50%, 'rectangle.css' is applied. If it is 50%, 'round.css' is applied.

As in the 'border-radius' property, the device-radius media feature can describe various round shapes of displays, such as rectangle, regular circle, ellipse, and rectangle with round corners.

Name:	device-radius
For:	@media
Value:	[ <length> \| <percentage> ]
Percentage:	Refer to corresponding dimension of the display
Type:	range

Note: To define a 'range' type media feature, the feature may be written as a normal media feature, but with a 'min-' or 'max-' prefix on the feature name. 'min-' or 'max-' prefixes express 'greater or equal to' or 'smaller or equal to' constraints respectively.

The length or percentage value of the 'device-radius' property defines a radius of a quarter ellipse in terms of the shape of the corner of the outer screen edge (This is similar to the 'border-radius' property. See the border-radius description). If the length is zero, the shape of screen is a rectangle, otherwise it is a rectangle with rounded corners or a circle or an ellipse. A percentage value of the 'device-radius' is proportional to a width and a height of the screen, which have relevance to a horizontal radius and a vertical radius of the screen according to the length of width and height of the screen each. If the screen shape is the regular circle, 'device-radius': 50% has a true value since a half of the width(=height) of the regular circle is the radius of the screen shape. In case of an ellipse with 200x100, 'device-radius': 50% means that horizontal-radius of the screen is 100px and vertical-radius of the screen is 50px. It can’t be described in length, not percentage. A negative value is not allowed.

The example below shows how a web page looks in the different shapes of displays. This is a simple clock written in HTML. (without device-radius)

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="rectangle.css" />
</head>
<body>
    <div id="clockLayer">
        <div id="clockLayer">
            <div id="date">2015/02/28 (SAT)</div>
            <div id="time">10:11</div>
            <div id="weather"><img src="cloudy.png" /></div>
        </div>
    </div>
</body>
</html>

(A) Rectangle Display

(B) Round Display

Devices where the device-radius media feature is not applicable

On the other hand, the example below shows how the device-radius media feature works in the different shapes of displays. This is the same as the code above except for media queries usage. The device-radius media feature can be used as follows:

<!-- index.html -->
<head>
    <link media="screen and (device-radius: 0%)" rel="stylesheet" href="rectangle.css" />
    <link media="screen and (device-radius: 50%)" rel="stylesheet" href="round.css" />
</head>

If this example code is loaded in a regular round display, 'round.css' will be applied by the media queries mechanism. To render the clock properly, 'round.css' could be written as follows.

#clockLayer {
    border-radius: 50%;
}
#clockInset {
    border-radius: 50%;
}
#date {
    text-align: center;
}
...

(A) Rectangle Display
(w/ device-radius: 0%)

An image of a round clock within a round display

(B) Round Display
(w/ device-radius: 50%)

Devices where the device-radius media feature is applicable

Note: For other shapes, such as polygons, we need to extend the media features with additional parameters. The current features have limitations to support the diversity beyond round shapes. How can we express star-shaped polygons? (e.g. SVG syntax, etc.) Of course, there is a trade-off between simplicity and expressiveness.

Using device-radius with just a single value (e.g. device-radius: 50%) is enough (for simplicity)? Otherwise, do we need to support all possible values the same as border-radius?

4. Aligning content along the display border

4.1. The shape-inside property

CSS Shapes [CSS-SHAPES] define the shape-inside property that aligns contents along the edge of a possibly non-rectangular wrapping area. Web authors may use this feature to fit contents inside a round display. However, it can be challenging to specify the wrapping area to be identical to the shape of a display. To address such cases, shape-inside is extended with a new value named 'display', such an element having this value will have its content (or contained elements) aligned along the display border automatically.

Name:	shape-inside
Value:	auto \| outside-shape \| [ <basic-shape> \|\| shape-box ] \| <image> \| `display`
Initial:	auto
Applies to:	block-level elements
Inherited:	no
Percentages:	n/a
Media:	visual
Computed value:	computed lengths for <basic-shape>, the absolute URI for <uri>, otherwise as specified
Animatable:	as specified for <basic-shape>, otherwise no

The example below shows how the shape-inside property works when it is set to 'display'. Without using Media Queries, contents can be aligned within the display edge automatically.

<style>
    #container {
        shape-inside: display;
        // the same as circle(50% at 50%, 50%) in a regular round display
    }
    #green-box { float: left; }
    #blue-box { float: right; }
</style>
<div id="container">
    <p>
        Some inline content
        <img id="green-box" src="green-box.jpg" />
        with a float left and float right, in a
        <img id="blue-box" src="blue-box.jpg" />
        simple box with a circle shape-inside.
    </p>
</div>

A layout of web contents without shape-inside:display

(A) Without 'shape-inside'

A layout of web contents with shape-inside: display

(B) With 'shape-inside: display'

Align the content along the display border

This property is specially useful for complex shapes (e.g. curved, stelliform, polygonal), that wouldn’t be covered by <basic-shape> (i.e. circle() or ellipse()), allowing web authors to conveniently align contents with the display edge.

When a containing block is placed on one end of the display and the containing block has 'shape-inside: display', the descendant blocks of the containing block are basically put on the overlapping region between the containing block and the display area. The overlapping region’s shape is mostly complicated shape, so it’s difficult to define the shape using previous method like basic-shape. The figure 4 describes these circumstances as follows.

An image of two examples to show the principle of shape-inside: display

Align part of the content along the display border

What if content overflows? Clipping or scrolling?

5. Drawing borders around the display border

5.1. The border-boundary property

We add the border-boundary property to set a boundary constraint that affects the borders of an element.

Name:	border-boundary
Value:	none \| parent \| display
Initial:	none
Applies to:	all elements
Inherited:	yes
Percentages:	n/a
Media:	visual
Computed value:	as specified
Animatable:	no

When the border-boundary property on an element is set to 'parent', additional borders of the element could be drawn where the element’s area and the borders of its parent are met. When it is set to 'display', additional borders could be drawn where the element’s area and the borders of screen are met. The default value is 'none', imposing no boundary constraint on the borders.

The example below shows how the border-boundary property works on drawing borders. The result is shown in Figure 5B.

<style>
    #container {
        border-boundary: display;
    }
    #redBox {
        border: 5px red solid;
    }
    #greenBox {
        border: 5px green solid;
    }
    #blueBox {
        border: 5px blue solid;
    }
</style>
<div id="container">
    <div id="redBox"></div>
    <div id="greenBox"></div>
    <div id="blueBox"></div>
</div>

An image of circle drawing border lines without border-boundary: display

(A) Without 'border-boundary'

An image of circle drawing border lines with border-boundary: display

(B) With 'border-boundary: display'

Align the content along the display border

Note: If the value of border-boundary is parent or display, border lines of the element are actually just a visual effect. It triggers a layout for rendering in a general way, but in the above cases (border-boundary: parent|display), the layout doesn’t occur and it only draws the border lines inward from the containing block’s borders. With this situation, the borders might hide contents around the display edge.

6. Content positioning using polar coordinate system

Polar coordinate system is a two-dimensional coordinate system that describes the position of a point in a plane with a distance from a reference point and an angle from a reference direction. Elements could be placed along a circle or concentric circles, and the polar coordinate system is useful to handle such cases.

This section introduces polar positioning to support layout of elements in the polar coordinate system where the position of an element is determined by a distance from the center point within the containing element and an angle from the Y-axis.

We add 'polar' to the value of the position property to support the polar coordinates of an element itself. The polar-angle and polar-distance properties specify the position of an element.

Note: In polar coordinate system, a pole is the reference point and points are described as been a certain distance from it, as also a certain angle from the polar axis. In mathematical theory, the polar axis is commonly defined as the positive direction of the x-axis, but we consider the polar axis as the positive direction of the y-axis position as other CSS specifications usually do. Therefore, when the polar-angle value of an element is 0, the element is positioned on the y-axis. If the angle value of an element increases in the positive direction from 0, the element moves clockwise. The method to determine a direction using polar-angle works the same way in [CSS-VALUES], <angle> value.

6.1. The position property

When the position property on an element is set to 'polar', the element could be affected in polar coordinate system.

Name:	position
Value:	static \| relative \| absolute \| sticky \| fixed \| `polar`
Initial:	static
Applies to:	all elements except table-column-group and table-column
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	specified value
Animatable:	no

Note: The center point of the containing block of an element indicates the origin of polar coordinate when the value of position property of the element is 'polar'.

In conventional coordinate system used in the web, the origin is positioned in the upper left corner of a containing block. In comparison, the origin of polar coordinates is the center point of a containing block. Therefore different methods are required to deploy and transform elements, or set margin/padding values when using polar coordinates.

An example as below shows the difference between the conventional coordinates and the polar coordinates when positioning elements.

The codes below show the difference between conventional coordinate system in the web and polar coordinate system when positioning elements.

Figure 6A might be the result of

<body>
  <div style="position: absolute; width: 100px; height: 100px; background-color: blue;"></div>
  <div style="position: absolute; width: 50px; height: 50px; background-color: green;"></div>
</body>

Figure 6B might be the result of

<body>
  <div style="position: polar; width: 100px; height: 100px; background-color: blue;"></div>
  <div style="position: polar; width: 50px; height: 50px; background-color: green;"></div>
</body>

An image aligning two elements to containing block in the conventional coordinate system

(A) With 'position: absolute'

An image aligning two elements to containing block in the polar coordinate system

(B) With 'position: polar'

Positioning elements in conventional coordinate system and polar coordinate system

6.2. The polar-origin property

The polar-origin property sets a point of origin for polar coordinate system.

Not only elements are positioned within the containing block according to the origin but also properties such as display, margin, and padding are affected by the origin. In conventional coordinate system, the origin of coordinate system implicitly set to upper left corner of the containing block. But in polar coordinates, it is general to specify the point of origin as the center point of the containing block. Also, there would be a situation that the other point in the containing block area need to be the origin.

Name:	polar-origin
Value:	<position>
Initial:	50% 50%
Applies to:	all elements
Inherited:	no
Percentages:	Refer to the size of containing block
Media:	visual
Computed value:	as specified
Animatable:	as length, percentage, or calc

Where

Values are defined as follows:

<percentage>: A percentage for the horizontal offset is relative to the width of the containing block. A percentage for the vertical offset is relative to height of the containing block.

<length>: A length value gives a fixed length as the offset. The value for the horizontal and vertical offset represent an offset from the top left corner of the containing block.

top: Computes to 0% for the vertical position.

right: Computes to 100% for the horizontal position.

bottom: Computes to 100% for the vertical position.

left: Computes to 0% for the horizontal position.

center: Computes to 50% (left 50%) for the horizontal position if the horizontal position is not otherwise specified, or 50% (top 50%) for the vertical position if it is.

Between the item and the containing block, which is appropriate for polar-origin value to be referenced of?

As it’s available to specify the point of origin for polar coordinates, properties such as margin, padding would work different from the conventional coordinate system. We need different approaches to define those properties which are related to layout.

6.3. The polar-angle property

The polar-angle property specifies the angle from the Y-axis. This property is activated on condition of position: polar.

Name:	polar-angle
Value:	<angle>
Initial:	0
Applies to:	all elements
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	as specified
Animatable:	as angle

6.4. The polar-distance property

The polar-distance property determines how far elements are positioned from the origin of polar coordinates. This property is activated on condition of position: polar.

Name:	polar-distance
Value:	[ <length> \| <percentage> ] && contain?
Initial:	0
Applies to:	all elements
Inherited:	no
Percentages:	relative to distance from the origin of polar coordinates to edge of containing block
Media:	visual
Computed value:	as specified
Animatable:	as length, percentage, or calc

The polar-distance specifies the distance between the origin of polar coordinates and the anchor point of the element.

Values have the following meanings:

<length>: A length value gives a fixed length between the origin of polar coordinates and the anchor point of element.

<percentage>: A percentage is relative to the distance from a point of contact which is made by the edge of containing block’s shape and the gradient-line to the origin of polar coordinates. The starting point of the gradient-line is the origin of polar coordinates. And the value of the gradient of the line is polar angle value.

contain: Adjust polar-distance value of the positioned element which overflows shape of the containing block. The main purpose of this value is avoiding overflow when positioning elements. When overflowing occurs, the distance between the point of origin of polar coordinates and the anchor point of the element is reduced, until there are 2 contact points or less between edge of shape of containing block and content block of the element.

Here are some examples. The first example shows positioning elements with polar-distance not using extent keyword value. Some parts of elements are outside the boundary of the containing block’s shape:

<body>
  <div style="position: polar; polar-angle: 45deg; polar-distance: 100%"></div>
  <div style="position: polar; polar-angle: 180deg; polar-distance: 100%"></div>
</body>

In the second example, contain, the extent keyword value is added to the polar-distance value of each element to avoid overflowing.

<body>
  <div style="position: polar; polar-angle: 45deg; polar-distance: 100% contain"></div>
  <div style="position: polar; polar-angle: 180deg; polar-distance: 100% contain"></div>
</body>

Would polar-distance property need auto value?

This example shows a way to align elements within the polar coordinate system.

<body>
  <div id="circle1" style="position: polar; polar-angle: 0deg; polar-distance: 50%"></div>
  <div id="circle2" style="position: polar; polar-angle: 90deg; polar-distance: 20%"></div>
  <div id="circle3" style="position: polar; polar-angle: 225deg; polar-distance: 100%"></div>
</body>

An image of three elements positioned to polar coordinates

An example of polar positioning

How to rotate the element itself with an axis from the element position to the central point of containing block? Is new property needed to support that at one time in polar coordinate system?

How to position items to the edge of the containing block without overflowing it? (polar-anchor could be suggested)

One of the profits of polar coordinate system is performance improvement. The significant performance improvement can be seen in animation using the polar coordinates. How to define animation in the polar coordinates? Is there any difference between the conventional coordinates and the polar coordinates when animate elements?

By polar-distance property, a position of an element is specified based on the containing block’s center. In other way, is the method of positioning elements by the distance based on the edge of the containing block needed?

6.5. The polar-anchor property

The polar-anchor property sets an anchor point of the element. The anchor point specifies a position which is a representative point of the element. The anchor point could be set as any point within a content area of the element rather than being positioned to the upper left corner of the element by CSS box model.

With position: polar, the value of polar-distance is given to the distance between an anchor point and a center point of a containing block.

Name:	polar-anchor
Value:	<position>
Initial:	50% 50%
Applies to:	all elements
Inherited:	no
Percentages:	relative to width and height of an element
Media:	visual
Computed value:	as specified
Animatable:	as <position>

Meanings of some keywords in <position> are different from those used in polar-origin property.

<percentage>: A percentage for the horizontal offset is relative to width of content box area of the element. A percentage for the vertical offset is relative to height of content box area of the element. For example, with a value pair of '100%, 0%', an anchor point is on the upper right corner of the element.

<length>: A length value gives a length offset from the upper left corner of the element’s content area.

Only with polar-angle and polar-distance to position elements, adjusting values of those properties for avoiding elements sticking out of the containing block is required. When the appropriate anchor point is given, there is no need to adjust polar-distance value to avoid overflowing when positioning elements in the containing block.

This example shows an alignment of four elements with different anchor points positioned in a containing block.

<style>
  #item1 {
    position: polar;
    polar-angle: 45deg;
    polar-distance: 100%;
    polar-anchor: right top;
  }
  #item2 {
    position: polar;
    polar-angle: 135deg;
    polar-distance: 100%;
    polar-anchor: right bottom;
  }
  #item3 {
    position: polar;
    polar-angle: 225deg;
    polar-distance: 100%;
    polar-anchor: left bottom;
  }
  #item4 {
    position: polar;
    polar-angle: 315deg;
    polar-distance: 100%;
    polar-anchor: left top;
  }
</style>
<body>
  <div id="item1"></div>
  <div id="item2"></div>
  <div id="item3"></div>
  <div id="item4"></div>
</body>

An image of four elements with different anchor points positioned in a containing block

An example of polar-anchor

Is 'auto' needed for value of polar-anchor?

How to solve overflowing when an element has polar-anchor value as 'top', polar-angle value is 0deg, and polar-distance is equal to the distance from center to edge of containing block?

6.6. 2D Rotation Transform Function for self-rotating elements in polar coordinates

When elements are positioned in polar coordinates, there are many usecases which show the elements rotated toward the origin of polar coordinates. In such cases, degree of rotation has to be specified in the 2d rotation function for each element. But when using the keyword value instead of the <angle> value, the calculation of accurate rotation degree may be omitted.

The two-dimensional rotation function 'rotate()' used in polar coordinates is extended as follows:

rotate() = rotate( <angle> | polar-angle | polar-angle-reverse)

Values have the following meanings:

polar-angle: Rotate an element by the value of polar-angle property.

polar-angle-reverse: Rotate an element by the angle of the polar-angle property plus 180 degrees.

This example shows how the keyword value works in the 2d rotation function.

<style>
  #item1 {
    position: polar;
    polar-angle: 0deg;
    polar-distance: 90%;
    transform: rotate(polar-angle);
  }
  #item2 {
    position: polar;
    polar-angle: 45deg;
    polar-distance: 90%;
    transform: rotate(polar-angle);
  }
  #item3 {
    position: polar;
    polar-angle: 135deg;
    polar-distance: 90%;
    transform: rotate(polar-angle-reverse);
  }
  #item4 {
    position: polar;
    polar-angle: 180deg;
    polar-distance: 90%;
    transform: rotate(polar-angle-reverse);
  }
  #item5 {
    position: polar;
    polar-angle: 225deg;
    polar-distance: 90%;
    transform: rotate(polar-angle-reverse);
  }
  #item6 {
    position: polar;
    polar-angle: -45deg;
    polar-distance: 90%;
    transform: rotate(polar-angle);
  }
</style>
<body>
  <div id="item1">1</div>
  <div id="item2">2</div>
  <div id="item3">3</div>
  <div id="item4">4</div>
  <div id="item5">5</div>
  <div id="item6">6</div>
</body>

An example of the extension for 2d rotate function

If this function is used inside an animation, which event is expected to occur?

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:

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

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.

CSS Round Display Level 1

Editor’s Draft, 20 November 2015

Abstract

Status of this document

Table of Contents