CSS Device Adaptation

Abstract

This specification provides a way for an author to specify, in CSS, the size, zoom factor, and orientation of the viewport that is used as the base for the initial containing block.

Status of this document

This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-device-adapt” in the subject, preferably like this: “[css-device-adapt] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

• 1. Introduction
• 2. Values
• 3. The viewport
• 4. The @viewport rule
  • 4.1. Syntax
• 5. Viewport descriptors
  • 5.1. The ‘min-width’ and ‘max-width’ descriptors
  • 5.2. The ‘width’ shorthand descriptor
  • 5.3. The ‘min-height’ and ‘max-height’ descriptor
  • 5.4. The ‘height’ shorthand descriptor
  • 5.5. The ‘zoom’ descriptor
  • 5.6. The ‘min-zoom’ descriptor
  • 5.7. The ‘max-zoom’ descriptor
  • 5.8. The ‘user-zoom’ descriptor
  • 5.9. The ‘orientation’ descriptor
• 6. Constraining viewport descriptor values
  • 6.1. Definitions
  • 6.2. The procedure
• 7. Media Queries
• 8. CSSOM
• 9. DOM Interfaces
• 10. Conformance
• 11. Viewport META element
  • 11.1. Properties
  • 11.2. Parsing algorithm
  • 11.3. ‘extend-to-zoom’
  • 11.4. Translation into @viewport descriptors
• 12. Handling ‘auto’ for ‘zoom’
• 13. UA stylesheets
  • 13.1. Desktop UA styles
  • 13.2. Small screen UA styles
• Acknowledgments
• References
  • Normative references
  • Other references
• Descriptor index
• Index

1. Introduction

This section is not normative.

CSS 2.1 [CSS21] specifies an initial containing block for continuous media that has the dimensions of the viewport. Mobile/handheld device browsers have a viewport that is generally a lot narrower than a desktop browser window at a zoom level that gives a CSS pixel size recommended by CSS 2.1.

The narrow viewport is a problem for documents designed to look good in desktop browsers. The result is that mobile browser vendors use a fixed initial containing block size that is different from the viewport size, and close to that of a typical desktop browser window. In addition to scrolling or panning, zooming is often used to change between an overview of the document and zoom in on particular areas of the document to read and interact with.

Certain DOCTYPEs (for instance XHTML Mobile Profile) are used to recognize mobile documents which are assumed to be designed for handheld devices, hence using the viewport size as the initial containing block size.

Additionally, an HTML META tag has been introduced for allowing an author to specify the size of the initial containing block, and the initial zoom factor directly. It was first implemented by Apple for the Safari/iPhone browser, but has since been implemented for the Opera, Android, and Fennec browsers. These implementations are not fully interoperable and this specification is an attempt at standardizing the functionality provided by the viewport META tag in CSS.

2. Values

This specification follows the CSS property definition conventions from [CSS3SYN].

Value types are defined in [CSS3VAL].

3. The viewport

In CSS 2.1 a viewport is a feature of a user agent for continuous media and used to establish the initial containing block for continuous media. For paged media, the initial containing block is based on the page area. The page area can be set through @page rules. Hence, @viewport applies to continuous media, and @page to paged media, and they will not interact or conflict.

This specification introduces a way of overriding the size of the viewport provided by the user agent (UA). Because of this, we need to introduce the difference between the initial viewport and the actual viewport.

initial viewport

This refers to the viewport before any UA or author styles have overridden the viewport given by the window or viewing area of the UA. Note that the initial viewport size will change with the size of the window or viewing area.

actual viewport

This is the viewport you get after the cascaded viewport descriptors, and the following constraining procedure have been applied.

When the actual viewport cannot fit inside the window or viewing area, either because the actual viewport is larger than the initial viewport or the zoom factor causes only parts of the actual viewport to be visible, the UA should offer a scrolling or panning mechanism.

It is recommended that initially the upper-left corners of the actual viewport and the window or viewing area are aligned if the base direction of the document is ltr. Similarly, that the upper-right corners are aligned when the base direction is rtl. The base direction for a document is defined as the computed value of the direction property for the first BODY element of an HTML or XHTML document. For other document types, it is the computed direction for the root element.

"dbaron: The question is, what does this do on the desktop browser? (And what's a desktop browser)". Need to say that a "desktop" browser typically have no UA styles, as opposed to the UA stylesheet outlined for current mobile behaviour, and that no UA styles for @viewport will give "desktop" behaviour per default (actual viewport is initial viewport).

4. The @viewport rule

The @viewport at-rule consists of the @-keyword followed by a block of descriptors describing the viewport.

The descriptors inside an @viewport rule are per document and there is no inheritance involved. Hence declarations using the ‘inherit’ keyword will be dropped. They work similarly to @page descriptors and follow the cascading order of CSS. Hence, descriptors in @viewport rules will override descriptors from preceding rules. The declarations allow !important which will affect cascading of descriptors accordingly.

Should the @viewport rule apply to top-level documents only? If not, we need to say something about different zoom factors in frames.

Bert: What's interactions of @viewport and @page

@viewport {
  width: device-width;
}

4.1. Syntax

The syntax for the @viewport rule is as follows (using the notation from the Grammar appendix of CSS 2.1 [CSS21]):

viewport
  : VIEWPORT_SYM S*
    '{' S* declaration? [ ';' S* declaration? ]* '}' S*;

with the new token:

@{V}{I}{E}{W}{P}{O}{R}{T} {return VIEWPORT_SYM;}

where:

V               v|\\0{0,4}(56|76)(\r\n|[ \t\r\n\f])?|\\v
W               w|\\0{0,4}(57|77)(\r\n|[ \t\r\n\f])?|\\w

The viewport non-terminal is added to the stylesheet production along with the ruleset, media, and page non-terminals:

stylesheet
  : [ CHARSET_SYM STRING ';' ]?
    [S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
    [ [ ruleset | media | page | viewport ] [ CDO S* | CDC S* ]* ]*
  ;

It is also added to media production to allow @viewport rules nested inside @media rules This is extending the CSS 2.1 syntax. A draft of CSS3 Paged Media also allows page inside @media.:

media
  : MEDIA_SYM S* media_list LBRACE S* [ ruleset | viewport ]* '}' S*
  ;

5. Viewport descriptors

This section presents the descriptors that are allowed inside an @viewport rule. Other descriptors than those listed here will be dropped.

Relative length values are resolved against initial values. For instance ‘em’s are resolved against the initial value of the font-size property. Viewport lengths (vw, vh, vmin, vmax) are relative to the initial viewport.

5.1. The ‘min-width’ and ‘max-width’ descriptors

Specifies the minimum and maximum width of the viewport that is used to set the size of the initial containing block where

<viewport-length> = auto | device-width | device-height | <length> | <percentage>

and the values have the following meanings:

‘auto’

The used value is calculated from the other descriptors' values according to the constraining procedure.

‘device-width’

The width of the screen in CSS pixels at zoom factor 1.0.

‘device-height’

The height of the screen in CSS pixels at zoom factor 1.0.

<length>

A positive absolute or relative length.

<percentage>

A percentage value relative to the width or height of the initial viewport at zoom factor 1.0, for horizontal and vertical lengths respectively. Must be positive.

The min-width and max-width descriptors are inputs to the constraining procedure. The width will initially be set as close as possible to the initial viewport width within the min/max constraints.

5.2. The ‘width’ shorthand descriptor

This is a shorthand descriptor for setting both min-width and max-width. One <viewport-length> value will set both min-width and max-width to that value. Two <viewport-length> values will set min-width to the first and max-width to the second.

5.3. The ‘min-height’ and ‘max-height’ descriptor

Specifies the minimum and maximum height of the viewport that is used to set the size of the initial containing block.

The min-height and max-height descriptors are inputs to the constraining procedure. The height will initially be set as close as possible to the initial viewport height within the min/max constraints.

5.4. The ‘height’ shorthand descriptor

This is a shorthand descriptor for setting both min-height and max-height. One <viewport-length> value will set both min-height and max-height to that value. Two <viewport-length> values will set min-height to the first and max-height to the second.

5.5. The ‘zoom’ descriptor

Specifies the initial zoom factor for the window or viewing area. This is a magnifying glass type of zoom. Interactively changing the zoom factor from the initial zoom factor does not affect the size of the initial or the actual viewport.

Values have the following meanings: Should both numbers and percentages be allowed?

‘auto’

The zoom factor is UA-dependent. The UA may use the size of the area of the canvas on which the document is rendered to find that initial zoom factor. See this section for a proposed way of handling ‘auto’ values for ‘zoom’.

<number>

A positive number used as a zoom factor. A factor of 1.0 means that no zooming is done. Values larger than 1.0 gives a zoomed-in effect and values smaller than 1.0 a zoomed-out effect.

<percentage>

A positive percentage value used as a zoom factor. A factor of 100% means that no zooming is done. Values larger than 100% gives a zoomed-in effect and values smaller than 100% a zoomed-out effect.

5.6. The ‘min-zoom’ descriptor

Specifies the smallest allowed zoom factor. It is used as input to the constraining procedure to constrain non-‘auto’ ‘zoom’ values, but also to limit the allowed zoom factor that can be set through user interaction. The UA should also use this value as a constraint when choosing an actual zoom factor when the used value of ‘zoom’ is ‘auto’.

Values have the following meanings:

‘auto’

The lower limit on zoom factor is UA dependant. There will be no minimum value constraint on the ‘zoom’ descriptor used in the constraining procedure

<number>

A positive number limiting the minimum value of the zoom factor.

<percentage>

A positive percentage limiting the minimum value of the zoom factor.

5.7. The ‘max-zoom’ descriptor

Specifies the largest allowed zoom factor. It is used as input to the constraining procedure to constrain non-‘auto’ ‘zoom’ values, but also to limit the allowed zoom factor that can be set through user interaction. The UA should also use this value as a constraint when choosing an actual zoom factor when the used value of ‘zoom’ is ‘auto’.

Values have the following meanings:

‘auto’

The upper limit on zoom factor is UA dependant. There will be no maximum value constraint on the ‘zoom’ descriptor used in the constraining procedure

<number>

A positive number limiting the maximum value of the zoom factor.

<percentage>

A positive percentage limiting the maximum value of the zoom factor.

5.8. The ‘user-zoom’ descriptor

Specifies if the zoom factor can be changed by user interaction or not.

Values have the following meanings:

‘zoom’

The user can interactively change the zoom factor.

‘fixed’

The user cannot interactively change the zoom factor.

5.9. The ‘orientation’ descriptor

This descriptor is used to request that a document is displayed in portrait or landscape mode. For a UA/device where the orientation is changed upon tilting the device, an author can use this descriptor to inhibit the orientation change. The descriptor should be respected for standalone web applications, and when the document is displayed in fullscreen. It is recommended that it is ignored for normal web navigation to avoid confusing the user.

Values have the following meanings:

‘auto’

The UA automatically chooses the orientation based on the device's normal mode of operation. The UA may choose to change the orientation of the presentation when the device is tilted.

‘portrait’

The document should be locked to portrait presentation.

‘landscape’

The document should be locked to landscape presentation.

6. Constraining viewport descriptor values

6.1. Definitions

For the procedure below:

Descriptors refer to the values resolved/constrained to at that point in the procedure. They are initially resolved to their computed values.

width and height refer to the resolved viewport size and not the shorthand descriptors. They are both initially ‘auto’.

MIN/MAX computations where one of the arguments is ‘auto’ resolve to the other argument. For instance, MIN(0.25, 'auto') = 0.25, and MAX(5, 'auto') = 5.

initial-width is the width of the initial viewport in pixels at zoom factor 1.0.

initial-height is the height of the initial viewport in pixels at zoom factor 1.0.

6.2. The procedure

The used values are resolved from the computed values going through the steps below.

User agents are expected, but not required, to re-run this procedure and re-layout the document, if necessary, in response to changes in the user environment, for example if the device is tilted from landscape to portrait mode or the window that forms the initial viewport is resized.

However, Media Queries and Device Adaption are tethered specifications. As a result, UAs that also support Media Queries must re-run this procedure and re-layout the document in all cases where changes in the user environment would cause them to re-evaluate Media Queries.

Resolve min-zoom and max-zoom values

1. If min-zoom is not ‘auto’ and max-zoom is not ‘auto’, set max-zoom = MAX(min-zoom, max-zoom)

Constrain zoom value to the [min-zoom, max-zoom] range

1. If zoom is not ‘auto’, set zoom = MAX(min-zoom, MIN(max-zoom, zoom))

Resolve non-‘auto’ lengths to pixel lengths

1. Resolve relative and absolute lengths, percentages, and keywords (‘device-width’, ‘device-height’) to pixel values for the ‘min-width’, ‘max-width’, ‘min-height’ and ‘max-height’ descriptors.

Resolve initial width and height from min/max descriptors

1. If min-width or max-width is not ‘auto’, set width = MAX(min-width, MIN(max-width, initial-width))
2. If min-height or max-height is not ‘auto’, set height = MAX(min-height, MIN(max-height, initial-height))

Resolve width value

1. If width and height are both ‘auto’, set width = initial-width
2. If width is ‘auto’, set width = height * (initial-width / initial-height)

Resolve height value

1. If height is ‘auto’, set height = width * (initial-height / initial-width)

7. Media Queries

For several media features, the size of the initial containing block and the orientation of the device affects the result of a media query evaluation, which means that the effect of @viewport rules on media queries needs extra attention.

Bert: If you put @viewport, can you put @viewport in @media? Say what it means?

From the Media Queries specification [MEDIAQ]:

“To avoid circular dependencies, it is never necessary to apply the style sheet in order to evaluate expressions. For example, the aspect ratio of a printed document may be influenced by a style sheet, but expressions involving ‘device-aspect-ratio’ will be based on the default aspect ratio of the user agent.”

For @viewport rules, though, the UA must cascade them separately with the initial viewport size used for evaluating media feature expressions and other values that depend on the viewport size to avoid circular dependencies, but use the actual viewport size when cascading all other rules.

Procedure for applying CSS rules:

1. Cascade all @viewport rules using the initial viewport size for values and evaluations which rely on viewport size
2. Compute the actual viewport from the cascaded viewport descriptors
3. Cascade all other rules using the actual viewport size

The rationale for using the viewport descriptors obtained from applying the @viewport rules for evaluating media queries for style rules, is that media queries should match the actual viewport that the document will be layed out in and not the initial or the one specified in the UA stylesheet. Consider the example below given that the UA stylesheet has a viewport width of 980px, but a device-width and initial viewport width of 320px. The author has made separate styles to make the document look good for initial containing block widths above or below 400px. The actual viewport used will be 320px wide, and in order to match the styles with the actual viewport width, the viewport resulting from applying the @viewport rules should be used to evaluate the media queries.

@viewport {
  width: device-width;
}

@media screen and (min-width: 400px) {
  div { color: red; }
}

@media screen and (max-width: 400px) {
  div { color: green; }
}

Another example:

@media screen and (width: 397px) {
  div { color: green; }
}

@viewport {
  width: 397px;
}

Below is an example where an @viewport rule relies on a media query affected by the viewport descriptors.

@viewport {
  width: 397px;
}

@media screen and (width: 397px) {
  @viewport {
    width: 500px;
  }
}

@media screen and (width: 397px) {
  div { color: green; }
}

It is recommended that authors do not write @viewport rules that rely on media queries whose evaluation is affected by viewport descriptors. Is is also recommended that the @viewport rule(s) is placed as early in the document as possible to avoid unnecessary re-evaluation of media queries or reflows.

8. CSSOM

Properties in the CSSOM and CSSOM View specifications refer to the viewport and the initial containing block. If any of those properties should refer to the initial viewport and not the actual viewport, those exceptions need to be adressed.

9. DOM Interfaces

The @viewport rule is exposed to the CSSOM through a new CSSRule interface

Interface CSSRule

The following rule type is added to the CSSRule interface. It provides identification for the new viewport rule.

IDL Definition

interface CSSRule {
    ...
    const unsigned short VIEWPORT_RULE = 15;
    ...
};

Interface CSSViewportRule

The CSSViewportRule interface represents the style rule for an @viewport rules

IDL Definition

interface CSSViewportRule : CSSRule {
  readonly attribute CSSStyleDeclaration style;
};

Attributes

style of type CSSStyleDeclaration

This attribute represents the viewport descriptors associated with this @viewport rule.

No Methods

10. Conformance

Requirements for a conforming UA:

• The ‘min-width’, ‘max-width’, ‘width’, ‘min-height’, ‘max-height’, and ‘height’ descriptors must be supported.

• The ‘min-zoom’, ‘max-zoom’, and ‘zoom’ descriptors must be supported as input to the constraining procedure. However, the UA may choose to use a different zoom factor when presenting the document to the user, and use different minimum and maximum zoom limits for the user interaction.

  This will for instance allow UAs without zooming capabilities to conform and still have interoperable implementations when it comes to viewport dimensions. It will also allow the UA to choose a different zoom factor when content overflows the actual viewport.

• Support for the ‘user-zoom’ and ‘orientation’ descriptors is optional.

11. Viewport META element

This section is not normative.

This section describes a mapping from the content attribute of the viewport META element, first implemented by Apple in the iPhone Safari browser, to the descriptors of the @viewport rule described in this specification.

In order to match the Safari implementation, the following parsing algorithm and translation rules rely on the UA stylesheet below. See the section on UA stylesheets for an elaborate description.

@viewport {
  width: extend-to-zoom 980px;
  min-zoom: 0.25;
  max-zoom: 5;
}

Note that these values might not fit well with all UAs. For instance, with a min-zoom of 0.25 you will be able to fit the whole width of the document inside the window for widths up to 1280px on a 320px wide device like the original iPhone, but only 960px if you have a 240px display (all widths being given in CSS pixel units).

11.1. Properties

The recognized properties in the viewport META element are:

• width
• height
• initial-scale
• minimum-scale
• maximum-scale
• user-scalable

11.2. Parsing algorithm

Below is an algorithm for parsing the content attribute of the META tag produced from testing Safari on the iPhone. The testing was done on an iPod touch running iPhone OS 4. The UA string of the browser: "Mozilla/5.0 (iPod; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7". The pseudo code notation used is based on the notation used in [Algorithms].

The whitespace class contains the following characters (ascii):

• Horizontal tab (0x09)
• Line feed (0x0a)
• Carriage return (0x0d)
• Space (0x20)

The recognized separator between property/value pairs is comma for the Safari implementation. Some implementations have supported both commas and semicolons. Because of that, existing content use semicolons instead of commas. Authors should be using comma in order to ensure content works as expected in all UAs, but implementors may add support for both to ensure interoperability for existing content.

The separator class contains the following characters (ascii), with comma as the preferred separator and semicolon as optional:

• Comma (0x2c)
• Semicolon (0x3b)

Parse-Content(S)
i ← 1
while i ≤ length[S]
    do while i ≤ length[S] and S[i] in [whitespace, separator, '=']
        do i ← i + 1
    if i ≤ length[S]
        then i ← Parse-Property(S, i)

Parse-Property(S, i)
start ← i
while i ≤ length[S] and S[i] not in [whitespace, separator, '=']
    do i ← i + 1
if i > length[S] or S[i] in [separator]
    then return i
property-name ← S[start .. (i - 1)]
while i ≤ length[S] and S[i] not in [separator, '=']
    do i ← i + 1
if i > length[S] or S[i] in [separator]
    then return i
while i ≤ length[S] and S[i] in [whitespace, '=']
    do i ← i + 1
if i > length[S] or S[i] in [separator]
    then return i
start ← i
while i ≤ length[S] and S[i] not in [whitespace, separator, '=']
    do i ← i + 1
property-value ← S[start .. (i - 1)]
Set-Property(property-name, property-value)
return i

Set-Property matches the listed property names case-insensitively. The property-value strings are interpreted as follows:

1. If a prefix of property-value can be converted to a number using strtod, the value will be that number. The remainder of the string is ignored.
2. If the value can not be converted to a number as described above, the whole property-value string will be matched with the following strings case-insensitively: yes, no, device-width, device-height
3. If the string did not match any of the known strings, the value is unknown.

11.3. ‘extend-to-zoom’

In order to be able to implement the functionality from META viewport where the viewport width or height is extended to fill the viewing area at a given zoom level, we introduce a UA internal value to the list of <viewport-length> values called ‘extend-to-zoom’ It will be used in width and height declarations in the translation outlined in the section below.

This new value is necessary in order to implement the mapping for two reasons. First, whether resolving the width/height needs to extend the pixel length to the visible width/height for a given zoom factor depends on the current initial width/height. <meta name="viewport" content="width=400, initial-scale=1"> yields a width of 400px for an initial-width of 320px, and 640px for an initial width of 640px. This can not be expressed as normative min/max descriptors that would constrain correctly when the initial width changes like for an orientation change.

Secondly, the extended width/height also relies on cascading viewport properties from different sources, including ‘min-zoom’ and ‘max-zoom’ from the UA stylesheet. For instance, if the UA stylesheet has max-zoom: 5, and the initial width is 320px, <meta name="viewport" content="width=10"> will resolve to 64px.

Resolving ‘extend-to-zoom’

The ‘extend-to-zoom’ value is resolved to pixel or auto lengths as part of step 3 of the constraining procedure. Since this is a non-normative descriptor value, the resolution is described here. Note that max-descriptors need to be resolved to pixel lengths before min-descriptors when ‘extend-to-zoom’ is a valid value.

Let extend-zoom = MIN(zoom, max-zoom)

For non-‘auto’ extend-zoom, let:

extend-width = initial-width / extend-zoom
extend-height = initial-height / extend-zoom

Then, resolve for ‘extend-to-zoom’ as follows:

• If extend-zoom is ‘auto’:
  1. If max-width is ‘extend-to-zoom’, set max-width = 'auto'
  2. If max-height is ‘extend-to-zoom’, set max-height = 'auto'
  3. If min-width is ‘extend-to-zoom’, set min-width = max-width
  4. If min-height is ‘extend-to-zoom’, set min-height = max-height
• If extend-zoom is non-‘auto’:
  1. If max-width is ‘extend-to-zoom’, set max-width = extend-width
  2. If max-height is ‘extend-to-zoom’, set max-height = extend-height
  3. If min-width is ‘extend-to-zoom’, set min-width = MAX(extend-width, max-width)
  4. If min-height is ‘extend-to-zoom’, set min-height = MAX(extend-height, max-height)

11.4. Translation into @viewport descriptors

The Viewport META element is placed in the cascade as if it was a STYLE element, in the exact same place in the dom, that only contains a single @viewport rule.

Each of the property/value pair from the parsing in the previous section are translated, and added to that single at-rule as follows:

Unknown properties

Unknown properties are dropped.

The width and height properties

The width and height viewport META properties are translated into ‘width’ and ‘height’ descriptors, setting the ‘min’ value to ‘extend-to-zoom’ and the ‘max’ value to the pixel length from the viewport META property.

1. Non-negative number values are translated to pixel lengths, clamped to the range: [1px, 10000px]
2. Negative number values are dropped
3. device-width and device-height are used as keywords
4. Other keywords and unknown values translate to 1px

<meta name="viewport" content="width=500, height=600">

@viewport {
  width: extend-to-zoom 500px;
  height: extend-to-zoom 600px;
}

For a viewport META element that translates into an @viewport rule with a non-‘auto’ ‘zoom’ declaration and no ‘width’ declaration:

• If it adds a ‘height’ descriptor, add:

  width: auto;

• Otherwise, add:

  width: extend-to-zoom;

to the @viewport rule.

<meta name="viewport" content="initial-scale=1.0">

@viewport {
  zoom: 1.0;
  width: extend-to-zoom;
}

<meta name="viewport" content="initial-scale=2.0, height=device-width">

@viewport {
  zoom: 2.0;
  width: auto;
  height: extend-to-zoom device-width;
}

The initial-scale, minimum-scale, and maximum-scale properties

The properties are translated into ‘zoom’, ‘min-zoom’, and ‘max-zoom’ respectively with the following translations of values.

1. Non-negative number values are translated to <number> values, clamped to the range [0.1, 10]
2. Negative number values are dropped
3. yes is translated to 1
4. device-width and device-height are translated to 10
5. no and unknown values are translated to 0.1

For a viewport META element that translates into an @viewport rule with no ‘max-zoom’ declaration and a non-‘auto’ ‘min-zoom’ value that is larger than the ‘max-zoom’ value of the UA stylesheet, the ‘min-zoom’ declaration value is clamped to the UA stylesheet ‘max-zoom’ value.

The user-scalable property

The user-scalable property is translated into ‘user-zoom’ with the following value translations.

1. yes and no are translated into ‘zoom’ and ‘fixed’ respectively.
2. Numbers ≥ 1, numbers ≤ -1, device-width and device-height are mapped to ‘zoom’
3. Numbers in the range <-1, 1>, and unknown values, are mapped to ‘fixed’

<meta name="viewport" content="width=480, initial-scale=2.0, user-scalable=1">

@viewport {
  width: 480px;
  zoom: 2.0;
  user-zoom: zoom;
}

12. Handling ‘auto’ for ‘zoom’

This section is not normative.

This section presents one way of picking an actual value for the ‘zoom’ descriptor when the used value is ‘auto’.

Given an initial viewport with size (initial-width, initial-height), and a finite region within the canvas where the formatting structure is rendered (rendered-width, rendered-height). That region is at least as large as the actual viewport.

Then, if the used value of ‘zoom’ is ‘auto’, let the actual zoom factor be:

zoom = MAX(initial-width / rendered-width, initial-height / rendered-height)

The actual zoom factor should also be further limited by the [min-zoom, max-zoom] range.

13. UA stylesheets

This section is informative

With the term desktop browser below, we mean a browser which has a size of the initial viewport, in CSS pixels, that is at least as large as the smallest viewport or viewing area you would expect a user of a desktop computer to have. In that sense, it could include tablet PC and TV browsers.

13.1. Desktop UA styles

For a desktop browser, the recommendation is to have no UA styles. That means that it will have all descriptors initially set to ‘auto’, and behave as it would have without support for viewport descriptors if there are no viewport descriptors in the user or author styles.

13.2. Small screen UA styles

For smaller screen UAs like smartphone browsers, the UA could typically set the minimum viewport width to something that is at least as large as the narrowest width an author would expect a desktop user would use to view documents.

@viewport {
  min-width: 980px;
}

It is recommended that limitations in zooming capabilities are not reflected in the UA styles but rather only affect the used values for zoom. The min-zoom/max-zoom UA styles mentioned in the Viewport META section are there to give an accurate description of how the Safari browser behaves, not as part of a recommended UA stylesheet.

Acknowledgments

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607/

[CSS3SYN]

L. David Baron. CSS3 module: Syntax. 13 August 2003. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2003/WD-css3-syntax-20030813

[CSS3VAL]

Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 28 August 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-values-20120828/

[MEDIAQ]

Florian Rivoal. Media Queries. 19 June 2012. W3C Recommendation. URL: http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/

Other references

[Algorithms]

Thomas H. Cormen; et al. Introduction to Algorithms, Second Edition, MIT Press.

Descriptor index

Index

• actual viewport, 3.
• auto
  • length, 5.1.
  • max-zoom, 5.7.
  • min-zoom, 5.6.
  • orientation, 5.9.
  • zoom, 5.5.
• device-height, 5.1.
  • viewport META, 11.2., ??, ??, ??
• device-width, 5.1.
  • viewport META, 11.2., ??, ??, ??
• fixed, 5.8.
• height, 5.4.
  • initial, 6.1.
  • resolved, 6.1.
  • viewport META, 11.1., ??, ??
• initial-scale, 11.1., ??
• initial viewport, 3.
• landscape, 5.9.
• max-height, 5.3.
• maximum-scale, 11.1., ??
• max-width, 5.1.
• max-zoom, 5.7.
• min-height, 5.3.
• minimum-scale, 11.1., ??
• min-width, 5.1.
• min-zoom, 5.6.
• no, 11.2., ??, ??
• orientation, 5.9.
• portrait, 5.9.
• Specifications whose evaluations are both affected by the same changes to the user environment, and so always must be evaluated together in order to ensure proper rendering., 6.2.
• user-scalable, 11.1., ??, ??
• user-zoom, 5.8.
• <viewport-length>, 5.1.
• width, 5.2.
  • initial, 6.1.
  • resolved, 6.1.
  • viewport META, 11.1., ??, ??
• yes, 11.2., ??, ??
• zoom, 5.5.
  • value, 5.8.