<pre class='metadata'>
Title: CSS Expressive Generalizations and Gadgetry Level 1
Shortname: css-egg
Level: 1
Status: ED
Work Status: Exploring
Group: csswg
Date: 2015-04-01
TR: https://www.w3.org/TR/css-egg/
ED: https://drafts.csswg.org/css-egg/
Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net, w3cid 43241
Abstract: This module extends the vocabulary of CSS with terms frequently used in certain domains, making it easier for authors to write understandable and maintainable style sheets.
</pre>

<pre class="link-defaults">
spec:css-images-3; type:value;
	text: closest-side
	text: farthest-side
	text: closest-corner
	text: farthest-corner
spec:css-values-3; type:type; text:<time>
spec:css-conditional-3; type:at-rule; text:@media
spec:css-values-4; type:value; text:s
</pre>

<style type="text/css">
body {
	background: -o-double-rainbow(center), url("https://www.w3.org/StyleSheets/TR/logo-[STATUS]") top left no-repeat;
	background: double-rainbow(center), url("https://www.w3.org/StyleSheets/TR/logo-[STATUS]") top left no-repeat;
}
:any-link {
	cursor: -o-double-rainbow(center), pointer;
	cursor: double-rainbow(center), pointer;
}
</style>

<h2 id="intro">
Introduction</h2>

	<em>This section is not normative.</em>

	As evidenced by the extraordinary variety of designs found on the web,
	CSS is a very expressive language,
	capable of describing unlimited styles and layouts.
	However, one of the design goal of CSS was also to make it
	easy to learn and to read by people.
	Making it simple for untrained authors to read and guess
	the meaning of a style sheet after only a basic introduction to CSS
	has contributed greatly to the success of the language.

	In addition, easy-to-read style sheets are also easier to maintain.

	In this spirit, this specification introduces a few new values and units,
	which, while not bringing significant new capabilities to the web platform,
	makes it possible to write style sheets that are easier to read than ever before,
	by enabling authors to use values and units that are familiar
	and appropriate for the domain they are working on.

<h3 id="interact">
Module Interactions</h3>

	This module extends:
	<ul>
		<li>the data type definitions in [[!CSS3-VALUES]]
		<li>the <<gradient>> definition in [[!CSS4-IMAGES]]
		<li>the 'voice-rate' property in [[!CSS3SPEECH]]
		<li>the '@media' rule in [[!MEDIAQUERIES-5]]
	</ul>

<h3 id="values">
Value Definitions</h3>

	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
	using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS3-VALUES]].
	Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS3-VALUES]].
	Combination with other CSS modules may expand the definitions of these value types.

	In addition to the property-specific values listed in their definitions,
	all properties defined in this specification
	also accept the <a>CSS-wide keywords</a> as their property value.
	For readability they have not been repeated explicitly.

<h2 id="units">
Extended Units</h2>
CSS provides several <a spec="css-values-3">absolute length</a> units such as ''cm'' or ''in'',
as well as several <<time>> units such as ''s'' or ''ms''
They can all be converted between each other at fixed ratios, but are
nonetheless provided in recognition that it is most convenient for authors
to use units that they are most familiar with, or that some designs are
more naturally expressed in some units than others.

<h3 id="astro-units">
Astronomical units</h3>
This specification adds four <a spec="css-values-3">absolute length</a> units
which should be appreciated by authors
in fields such as astronomy and fundamental physics.
The new units are ''ls'', ''pc''
and their sub multiples ''pls'' and ''apc''.
They are defined as <a spec="css-values-3">physical units</a>.

<table class="data" export>
	<thead>
		<tr><th>unit
		    <th>name
		    <th>equivalence
	<tbody>
		<tr><th><dfn>apc</dfn>
		    <td>attoparsec
		    <td>1 ''apc'' = 3.086 ''cm''
		<tr><th><dfn>pls</dfn>
		    <td>picolightsecond
		    <td>1 ''pls'' = 1.133 ''px''
		<tr><th><dfn>ls</dfn>
		    <td>lightsecond
		    <td>1 ''ls'' = 10<sup>12</sup> ''pls''
		<tr><th><dfn>pc</dfn>
		    <td>parsec
		    <td>1 ''pc'' = 10<sup>18</sup> ''apc''
</table>

Note: Lightsecond was chosen instead of the more familiar lightyear
to make sure that layouts computed during leap years
would be unambiguously identical
to those computed during non-leap years.

Note: This specification does not address relativistic effects
of scrolling or animations and transitions
at (or beyond) the speed of light.

<div class="example" id="celestial-css">
	When writing a page representing the inner solar system at scale, the following markup and style could be used.
	<pre><code class="lang-markup">
&lt;div id=space>
	&lt;div class=celestial-body id=sun data-color=yellow
		data-radius=2.3ls>
		&lt;div class=celestial-body id=mercury data-color=lightgray
			data-radius=0.0081ls data-semimajor-axis=193ls data-orbital-period=6.278ftn>&lt;/div>
		&lt;div class=celestial-body id=venus data-color=#FFD881
			data-radius=0.020ls data-semimajor-axis=361ls data-orbital-period=16.05ftn>&lt;/div>
		&lt;div class=celestial-body id=earth data-color=blue
			data-radius=0.021ls data-semimajor-axis=499ls data-orbital-period=26.09ftn>
			&lt;div class=celestial-body id=moon data-color=#E0E0E0
				data-radius=0.0057ls data-semimajor-axis=1.28ls data-orbital-period=2.109ftn>&lt;/div>
		&lt;/div>
		&lt;div class=celestial-body id=mars data-color=#AD674D
			data-radius=0.011ls data-semimajor-axis=760ls data-orbital-period=49.07ftn>&lt;/div>
&lt;/div>&lt;/div>
&lt;style></code><code class="lang-css">
#space {
	background:black;
	overflow:hidden;
	position: relative;
}
.celestial-body {
	position:absolute;
	left: 50%; top: 50%;
	border-radius: 50%;
	border-style:solid attr(data-radius length, 0) attr(data-color color, white);
	margin: calc(-1 * attr(data-radius length, 0));
	transform-origin: attr(data-radius length, 0)
	                  calc(attr(data-radius length, 0) + attr(data-semimajor-axis length, 0));
	animation: orbit linear infinite attr(data-orbital-period time, 0);
}
@keyframes orbit {
	from { transform: translateY(calc(attr(data-semimajor-axis length, 0) * -1)) rotate(0); }
	to { transform: translateY(calc(attr(data-semimajor-axis length, 0) * -1)) rotate(-360deg); }
}</code><code class="lang-markup">
&lt;/style></code>
	</pre>

	The following is a disappointingly small sample rendering (not at scale)
	of what this would look like.
	With the new units and more space than this specification can offer,
	this would instead be overwhelmingly awesome.
	<style>
/**
This would be nicer using attr() or custom properties,
but there's not enough support in browsers to make a good demo with them.

The proportions to make this fit in the spec are as follow:
1px = 1ls
1s = 10 days
sun radius x 2
planets & moon radius x 100
planetary orbits x 0.25
moon orbit x 5

All are rounded up to the closest integer pixel,
because otherwise it doesn't work right in some browsers.
 */
#sun {
	border-width: 5px;
	margin: -5px;
	border-color: yellow;
	top: 50%;
	box-shadow: yellow 0 0 10px, yellow 0 0 20px, yellow 0 0 30px;
	/* The box shadow size has no other justification than making the sun look good. */
}
#mercury {
	border-width: 1px;
	margin: -1px;
	top: -49px;
	transform-origin: 1px 50px;
	-webkit-transform-origin: 1px 50px;
	animation-duration: 8.797s;
	-webkit-animation-duration: 8.797s;
	border-color: lightgray;
}
#venus {
	border-width: 2px;
	margin: -2px;
	top: -90px;
	transform-origin: 2px 92px;
	-webkit-transform-origin: 2px 92px;
	animation-duration: 22.47s;
	-webkit-animation-duration: 22.47s;
	border-color: #FFD881;
}
#earth {
	border-width: 2px;
	margin: -2px;
	top: -125px;
	transform-origin: 2px 127px;
	-webkit-transform-origin: 2px 127px;
	animation-duration: 36.53s;
	-webkit-animation-duration: 36.53s;
	border-color: blue;
}
#moon {
	border-width: 1px;
	margin: -1px;
	top: -6px;
	transform-origin: 1px 7px;
	-webkit-transform-origin: 1px 7px;
	animation-duration: 2.953s;
	-webkit-animation-duration: 2.953s;
	border-color: #E0E0E0;
}
#mars {
	border-width: 1px;
	margin: -1px;
	top: -190px;
	transform-origin: 1px 191px;
	-webkit-transform-origin: 1px 191px;
	animation-duration: 68.7s;
	-webkit-animation-duration: 68.7s;
	border-color:#AD674D;
}
#space {
	height:400px;
	background:black;
	overflow:hidden;
	position: relative;
}
.celestial-body {
	left: 50%;
	position:absolute;
	border-style:solid;
	border-radius: 50%;
	animation: orbit linear infinite;
	-webkit-animation: orbit linear infinite;
}
@keyframes orbit {
	from { transform: rotate(0); }
	to { transform: rotate(-360deg); }
}
@-webkit-keyframes orbit {
	from { -webkit-transform: rotate(0); }
	to { -webkit-transform: rotate(-360deg); }
}
	</style>
	<div id=space>
		<div class=celestial-body id=sun>
			<div class=celestial-body id=mercury></div>
			<div class=celestial-body id=venus></div>
			<div class=celestial-body id=earth>
				<div class=celestial-body id=moon></div>
			</div>
			<div class=celestial-body id=mars></div>
		</div>
	</div>

</div>

<h3 id="traditional-time">
Traditional time units
</h3>

While the ''s'' and ''ms'' units defined in [[CSS3-VALUES]]
are appropriate for very short durations,
their use to express longer time spans is inadequate.
Moreover, authors may sometimes wish to use less dry terminology
and prefer more expressive and vivid language. This specification
therefore introduces two <<time>> units:
''ftn'' and its subdivision ''mftn''.

<table class="data" export>
	<thead>
		<tr><th>unit
		    <th>name
		    <th>equivalence
	<tbody>
		<tr><th><dfn>ftn</dfn>
		    <td>Fortnight
		    <td>1 ''ftn'' = 1209600 ''s''
		<tr><th><dfn>mftn</dfn>
		    <td>Millifortnight
		    <td>1 ''ftn'' = 1000 ''mftn''
</table>

<h3 id=speech>
Speech rate</h3>

This specification introduces an new <a spec="css-values">dimension</a>,
together with a new unit to be used with this dimension.
The <dfn id="speech-rate-value"><<speech-rate>></dfn> described the speed at which a text is read aloud.
It can be expressed using the ''tmbl'' unit, as defined below.

<table class="data" export>
	<thead>
		<tr><th>unit
		    <th>name
		    <th>definition
	<tbody>
		<tr><th><dfn>tmbl</dfn>
		    <td>Timble
		    <td>This is the rate of speech of Sir Tim Berners-Lee
		        when delivering a speech.
</table>

Note: Authors are advised to be cautious when using this unit,
as it can easily lead to rates of speech that are too high
for the listener to follow.
''1tmbl'' is generally considered to be about
the maximum rate a listener without special training can sustainably understand
while paying close attention.
Consider reducing the speed when the audience is distracted, tired,
or comprises many non-native speakers.
These factors are cumulative.
For dictation or during meetings for which live minutes are taken,
a rate of 0.3tmbl is more appropriate.

Issue: For languages other than English, should ''1tmbl'' be defined as
the same number of words per minute as ''1tmbl'' in English?
This seems problematic as we do not have a precise definition for <em>word</em>,
and because average word length can vary significantly per language.
Alternatively, defining ''1tmbl'' as
the speed at which Tim Berners-Lee speaks in that language
is also problematic,
given that there are languages that He does not speak at all.
Tim Berners-Lee has provided an alternative definition of the timble,
<a href="http://schepers.cc/timble">as documented by Doug Schepers</a>:
<q>speech at more than 1 timble is difficult to understand;
speech below 1 timble is simply boring</q>.
Doug himself defines it as
<q>the uppermost rate of speech
at which a normal person
can understand what’s being said in their native language</q>.

<h4 id="voice-rate-ext">
Extension to the 'voice-rate' property</h4>

This specification extends the 'voice-rate' property, so that the ''tmbl'' unit can be used.

<pre class="propdef partial" noexport>
    Name: voice-rate
    New values: [normal | x-slow | slow | medium | fast | x-fast | <<speech-rate>>] || &lt;percentage&gt;
</pre>

Issue: Should negative values be allowed for <<speech-rate>> for reversed speech?


<h3 id=fps>
Device performance</h3>

<div class="informative">
Device performance is known to be a perfectly linear quantity with a total ordering consistent across different kinds of workloads. It is desirable to be able to design adaptive content that works across a range of user agent and device performance.</div>

This specification introduces an new <a spec="css-values">dimension</a>, together with a new unit to be used with this dimension.

The <dfn id="device-performance-value"><<device-performance>></dfn> describes the performance of the current device and user agent combination. It can be expressed using the ''adafish'' unit, as defined below.


<table class="data" export>
	<thead>
		<tr><th>unit
		    <th>name
		    <th>definition
	<tbody>
		<tr><th><dfn>adafish</dfn>
		    <td>Adafish
		    <td>This is the maximum number of fish that can be displayed on the current device and user agent combination in <a href="https://ada.is/xrgarden/">Ada Rose Cannon's Fish Garden</a> without the user agent dropping any frames.
</table>

Note: <a href="https://ada.is/xrgarden/">Ada Rose Cannon's Fish Garden</a> supports changing the number of fish through the `?fish=` query parameter, e.g. <a href="https://ada.is/xrgarden/?fish=50">this link will show 50 fish</a>

<h4 id="media-query-ext">
Extension to the ''@media'' rule: The '@media/performance' feature</h4>

	<pre class='descdef mq'>
	Name: performance
	Value: <<device-performance>>
	For: @media
	Type: range
	</pre>

The '@media/performance' media feature describes the performance characteristics of the current device and user agent combination at the current time. It SHOULD incorporate transient and permanent environmental factors affecting performance such as the current room temperature, the accumulation of dust in the device's cooling system, and the existence of spilled lemonade or coffee on the device's motherboard.

When querying the '@media/performance' feature the user agent SHOULD ensure that it is querying a recent value. The user agent MAY in the background run <a href="https://ada.is/xrgarden/">Ada's Fish Garden</a> with various parameters to estimate this value. If device performance characteristics change while content is being viewed, the user agent MUST recompute its value for '@media/performance', potentially restyling the document.

<div class=example>
This feature is useful for designing robust content that adapts to different device characteristics.

<pre><code class="lang-css">
@media (min-performance: 1000adafish) {
	#very-cool-demo-with-blinky-lights {
		display: block;
	}
}

@media (max-performance: 1000adafish) {
	#really-bad-demo-with-no-blinky-lights-whatsoever {
		display: block;
	}
}
</code></pre>
</div>

<h4 id="adafish-fingerprinting">Fingerprinting concerns</h4>

This API may be used for gleaning information about the user's environment, which may be used to fingerprint them. This is recognized to be an acceptable loss for the momentous benefits this API is expected to provide the web platform.

<h3 id="random">Random values</h3>

[[CSS-VALUES-5]] introduces ''random()'' to generate random values.

As that function has very limited use cases,
this specification introduces three <dfn>random value units</dfn> covering a broader range of randomness.

<table class="data" export>
	<thead>
		<tr><th>unit
		    <th>name
		    <th>definition
	<tbody>
		<tr><th><dfn style="white-space: nowrap">🎲</dfn>
		    <td>Random number
		    <td>Returns a random <<number>> value between ''0'' and the given <<number>> value.
		<tr><th><dfn style="white-space: nowrap">🎲🎲</dfn>
		    <td>Random number and unit
		    <td>Returns a random <<dimension>> or <<percentage>> between ''0'' and the given value.
		<tr><th><dfn style="white-space: nowrap">❓</dfn>
		    <td>Random value
		    <td>Returns a <<number>>, <<dimension>>, or <<percentage>> between ''0'' and the given value,
		        a <<color>>, <<string>>, or any kind of <a href="https://drafts.csswg.org/css-values-4/#keywords">pre-defined keyword</a>.
		<tr><th><dfn style="white-space: nowrap">🎁</dfn>
		    <td>Complete randomness
		    <td>May return any kind of value defined in [[CSS-VALUES-5]],
		        including but not limited to <<number>>, <<dimension>>, or <<percentage>> values of any size,
		        a <<color>>, <<string>>, any kind of <<ident>>, or nothing at all.
		        It might even come up with new values not yet defined. Who knows?
</table>


<h2 id="rainbow">
Double Rainbow</h2>

<<gradient>> allows sophisticated visual effects,
but they are tedious to write, review and maintain
as non trivial gradients cannot easily be visualized by merely reading the source.
This specification introduces a new syntax to describe a particular kind of <<radial-gradient()>>: double rainbows.
This spectacular visual effect which would improve most web pages
is currently underused due to the difficulty of specifying it correctly.

The <<gradient>> syntax is extended to accept <<double-rainbow()>> in addition to the other values defined in [[!CSS4-IMAGES]]

<pre>
	<dfn>double-rainbow()</dfn> = double-rainbow(
		 <<position>> , [ <<extent>> | <<length-percentage [0,∞]>> ]?
	)
	<dfn noexport><<extent>></dfn> = ''closest-corner'' | ''closest-side'' | ''farthest-corner'' | ''farthest-side''
</pre>

<<length>> or <<percentage>> gives the radius of the outermost circle of the double rainbow explicitly.
Percentages values are relative to the corresponding dimension of the gradient box.
Negative values are invalid.

If the second argument is omitted, the default value is ''farthest-side''.

<div class="example" id="rainbow-unicorn">
This feature was initially introduced by Opera Software.
The following page, when viewed in Opera (between version 11.60 and 12.16),
demonstrates tasteful use of double rainbows:
<a href="http://media.opera.com/media/press/2011/unicorn/">http://media.opera.com/media/press/2011/unicorn/</a>

By combining ''double-rainbow()'' with <a href="https://fetch.spec.whatwg.org/#unicorn">about:unicorn</a> [[FETCH]],
modern web standards can be used to achieve a similar effect.
<pre><code class="lang-markup">
&lt;img src="about:unicorn"
style="background: double-rainbow(bottom, closest-corner)">
</code></pre>
<img class=fabulous src="images/unicorn.svg" no-autosize>
<style>
.fabulous {
background: radial-gradient(closest-corner circle at bottom, transparent, rgba(250, 250, 210, 0.251) 63%, rgba(148, 0, 211, 0.251), rgba(0, 0, 128, 0.251), rgba(0, 0, 255, 0.251), rgba(0, 128, 0, 0.251), rgba(255, 255, 0, 0.251), rgba(255, 165, 0, 0.251), rgba(255, 0, 0, 0.251), transparent 67%, transparent 90%, rgba(255, 0, 0, 0.126), rgba(255, 165, 0, 0.126), rgba(255, 255, 0, 0.126), rgba(0, 128, 0, 0.126), rgba(0, 0, 255, 0.126), rgba(0, 0, 128, 0.126), rgba(148, 0, 211, 0.126), transparent 100%);
max-width: 500px;
display:block;
margin-left: auto;
margin-right: auto;
}
</style>
</div>

<h2 class=no-num id="acknowledgments">
Acknowledgments</h2>

Thanks especially to the following people for their ideas, feedback, and experimental implementations (in alphabetical order by first name):
Anne van Kesteren,
Bruce Lawson,
Chris Mills,
Doug Schepers,
Daniel Glazman,
Leif Arne Storset,
Paul Verbeek,
Peter Linss,
Tim Berners-Lee (assuming the quotation by Doug correctly records what Tim said).
