[css-values] Define the concept of compatible and canonical units, and define canonical units for all relevant types.

Author: tabatkins

css-values/Overview.bs

@@ -24,6 +24,7 @@ Abstract: This CSS3 module describes the common values and units that CSS proper
 At Risk: ''toggle()'', ''attr()''
 Ignored Terms: <spacing-limit>, containing block
 Ignored Vars: Cn+1, n
+Inline Github Issues: yes
 </pre>
 <pre class='link-defaults'>
 spec: css-backgrounds-3; type: type; text: <position>
@@ -797,6 +798,15 @@ Numbers with Units: <a>dimensions</a></h3>
 	resolutions (<<resolution>>),
 	and other quantities.
 
+	When serializing <a lt="computed value">computed</a> and <a>used values</a>,
+	<dfn export local-lt=compatible>compatible units</dfn>
+	(those related by a static multiplicative factor,
+	like the 96:1 factor between ''px'' and ''in'',
+	or the 1000:1 factor between ''ms'' and ''s'')
+	are converted into a single <dfn export local-lt=canonical>canonical unit</dfn>.
+	Each group of compatible units defines which among them is the <a>canonical unit</a>
+	that will be used for serialization.
+
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@@ -1055,11 +1065,15 @@ Absolute lengths: the ''cm'', ''mm'', ''q'', ''in'', ''pt'', ''pc'', ''px'' unit
 		</pre>
 	</div>
 
+	All of the absolute length units are <a>compatible</a>,
+	and ''px'' is their <a>canonical unit</a>.
+
 	For a CSS device, these dimensions are anchored either
-	<ul>
-		<li>(i) by relating the <a>physical units</a> to their physical measurements, or
-		<li>(ii) by relating the <a value>pixel unit</a> to the <a>reference pixel</a>.
-	</ul>
+
+	<ol type=i>
+		<li>by relating the <a>physical units</a> to their physical measurements, or
+		<li>by relating the <a value>pixel unit</a> to the <a>reference pixel</a>.
+	</ol>
 
 	For print media and similar high-resolution devices,
 	the anchor unit should be one of the standard physical units (inches, centimeters, etc).
@@ -1175,6 +1189,9 @@ Angle Units: the <<angle>> type and ''deg'', ''grad'', ''rad'', ''turn'' units</
 	For example, a right angle is ''90deg'' or ''100grad'' or ''0.25turn'' or
 	approximately ''1.57rad''.
 
+	All <<angle>> units are <a>compatible</a>,
+	and ''deg'' is their <a>canonical unit</a>.
+
 	<div class="note">
 		By convention,
 		when an angle denotes a direction in CSS,
@@ -1211,6 +1228,9 @@ Duration Units: the <<time>> type and ''s'', ''ms'' units</h3>
 		<dd>Milliseconds. There are 1000 milliseconds in a second.
 	</dl>
 
+	All <<time>> units are <a>compatible</a>,
+	and ''s'' is their <a>canonical unit</a>.
+
 	Properties may restrict the time value to some range.
 	If the value is outside the allowed range,
 	the declaration is invalid and must be <a href="https://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
@@ -1241,6 +1261,9 @@ Frequency Units: the <<frequency>> type and ''Hz'', ''kHz'' units</h3>
 	For example, when representing sound pitches, 200Hz (or 200hz)
 	is a bass sound, and 6kHz (or 6khz) is a treble sound.
 
+	All <<frequency>> units are <a>compatible</a>,
+	and ''hz'' is their <a>canonical unit</a>.
+
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@@ -1273,6 +1296,11 @@ Resolution Units: the <<resolution>> type and ''dpi'', ''dpcm'', ''dppx'' units<
 	For uses, see e.g. the  ''resolution'' media query in [[MEDIAQ]]
 	or the 'image-resolution' property defined in [[CSS3-IMAGES]].
 
+	All <<resolution>> units are <a>compatible</a>,
+	and ''dpi'' is their <a>canonical unit</a>.
+
+	Issue(178):
+
 	Note: Due to the 1:96 fixed ratio of CSS ''in'' to CSS ''px'',
 	''1dppx'' is equivalent to ''96dpi''.
 	This corresponds to the default resolution of images displayed in CSS: see 'image-resolution'.