CSS 2.1 [CSS21] specifies an initial containing block for continuous media that has the dimensions of the viewport. Since the viewport is generally no larger than the display, devices with smaller displays such as phones or tablets typically present a smaller viewport than larger devices like desktop or laptops.
Unfortunately, many documents have historically been designed against larger viewports and exhibit a variety of bugs when viewed in smaller viewports. These include unintended layout wrapping, clipped content, awkward scrollable bounds, and script errors. To avoid these issues, mobile browsers generally use a fixed initial containing block width that mimics common desktop browser window size (typically 980-1024px). The resulting layout is then scaled down to fit in the available screen space.
Although this approach mitigates the issues mentioned above, the downscaling means the CSS pixel size will be smaller than recommended by CSS 2.1. Users will likely need to zoom on the content to view it comfortably.
This mitigation is unnecessary for sites that have been designed to work well on small viewports.
This specification is written from an implementation-centric point of view, making it arguably difficult to read. Significant editorial work may be needed to make it more understandable to different audiences. It also should clarify which viewport is referred to by various js APIs. See this blog post by ppk for a good discussion of these issues.
Various issues about this specification and related specifications are listed in this report.
2. The viewportIn 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.
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.
<meta> tag.
Make actual viewport the layout viewport, define visual viewport.
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.
segments. It can be a fold area of a device with a foldable screen or a physical split occupying a logical space within the viewport for example some dual screen models.
Below is an illustration describing the concept of display features and how they divide the viewport in segments:
The recognized properties in the viewport <meta> element are:
widthheightinitial-scaleminimum-scalemaximum-scaleuser-scalableinteractive-widgetBelow 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):
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:
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:
property-value can be converted to a number using strtod, the value will be that number. The remainder of the string is ignored.property-value string will be matched with the following strings case-insensitively: yes, no, device-width, device-heightSpecify extend-to-zoom behavior by the viewport meta tag
3.4. interactive-widgetMove the definition of visual viewport from CSSOM-View to this spec.
The interactive-widget property specifies the effect that interactive UI widgets have on the page’s viewports. It defines whether widgets overlay a given viewport or whether the viewport is shrunken so that it remains fully visible while the widget is showing. Interactive UI widgets are transient user agent or operating system UI through which a user can provide input.
The most common such UI widget is a virtual keyboard.
The following is a list of valid values for interactive-widget and the associated viewport-resizing behavior:
overlays-content
VirtualKeyboard.overlaysContent is set to true.
resizes-content
resizes-visual
If no value, or an invalid value, is set for interactive-widget, the behavior implied by resizes-visual is used as the default.
To resize a viewport by an interactive widget, subtract from it the intersection of the viewport rect with the widget’s OS reported bounding rect. In cases where this would result in a non-rectangular viewport, the behavior is user agent defined.
3.4.1. Interaction with virtualKeyboard.overlaysContent[VIRTUAL-KEYBOARD] provides an imperative API to apply the overlays-content behavior via the VirtualKeyboard.overlaysContent attribute. This attribute shadows the value set to interactive-widget, namely:
When VirtualKeyboard.overlaysContent is set to true, the UA MUST ignore any value set to interactive-widget when determining the resizing behavior of interactive widgets.
When VirtualKeyboard.overlaysContent is set to false, the UA MUST use the value set to interactive-widget, or the default behavior if a value is not set, when determining the resizing behavior of interactive widgets.
Getting the value of VirtualKeyboard.overlaysContent MUST return only the value previously set to it.
That is, unless previously set, VirtualKeyboard.overlaysContent returns false even if interactive-widget=overlays-content is set via the <meta> tag.
An element becomes zoomed when the zoom property has a positive computed value different than 1 (or when a flat tree ancestor has zoom).
To apply zoom, the used value of a CSS property (including values inside of path() strings) is pre-multiplied (before any other steps in the used value stage) by the used value of zoom for the element. It also multiplies the natural size of all replaced elements, background images, and nested frames (except for fenced frames [FENCED-FRAME]) by the used value of zoom.
Note: This results in a magnification or minification effect.
Note: Since this multiplication is on computed values, it applies to all inherited properties such as line-height and font-size.
Nested values of zoom multiply, resulting in additional scaling of <length> values. The used value for zoom is always its effective zoom.
The zoom property has no effect on <length> property values with computed values that are auto or <percentage>.
Note: Unlike transform, scaling the zoom property affects layout.
Note: The computed value of font-size is never <percentage>; thus zoom always applies.
Note: zoom does not affect or prevent transform scaling.
The values of this property have the following meanings:
Note: The treatment of 0 is a web compatibility quirk.
Note: The treatment of 0 is a web compatibility quirk.
Negative values for zoom are illegal.
Example of the
zoomproperty applied during hover for magnification effect.
<div class="messageBox">
<div class="label">Text of the label</div>
</div>
<style>
.messageBox {
width: 10em;
padding: 2em;
border: medium solid lightblue;
}
.messageBox:hover {
zoom: 150%;
}
.label {
background: lightgrey;
padding: 1em;
text-align: center;
}
</style>
Here is an llustration of the before and after hover state of the message box element:
Example of nested zoom. In this example, "Inner text" is 4x as large as "Outer text", and "Middle text" is 2x as large as "Outer text".
<div style="zoom: 2">
Middle text
<div style="zoom: 2">
Inner text
<div>
<div>
Outer text
Example of replaced elements. In this example, the image and iframe will be twice as large as their default sizing.
<div style="zoom: 2"> <img src="..."> <iframe src="..."></iframe> <div>
The effective zoom of an element is the product of its computed value of zoom and all flat tree ancestors' computed values of zoom.
The scaled value of a CSS length is the used value of that length; in particular it includes zoom.
The unscaled value of a CSS length relative to an element is the scaled value divided by the element’s effective zoom.
4.1. DOM and CSSOM interactionComputed style APIs (i.e., all values returned by getComputedStyle()) that are non-auto and non-percentage lengths must be unscaled.
The getBoundingClientRect, getClientRects, and IntersectionObserver APIs must return rects with scaled lengths.
All other APIs related to element geometries must return unscaled lengths (except as detailed below). This is explained in detail in CSSOM View § 7 Extensions to the HTMLElement Interface.
In cases where properties (such as scrollTop) are propagated to the viewport, APIs for these lengths must be in viewport units, and not be divided by the zoom of the element.
The devicePixelRatio of a frame is multiplied by the effective zoom inherited by its parent frame.
Window Interface
partial interface Window {
[SameObject, Replaceable] readonly attribute Viewport viewport;
};
6. Viewport 6.1. The Viewport Interface
[Exposed=Window]
interface Viewport {
readonly attribute FrozenArray<DOMRect>? segments;
};
7. The segments property
The segments property is an array of DOMRect that represent the dimensions of each existing viewport segment. Each DOMRect contains the geometry of the segment (x, y, width, height) in CSS px.
Additional details about the definition of a viewport segment can be found here: CSS Environment Variables 1 § 2.3 Viewport segment variables.
The segments attribute getter steps are:
If the Viewport’s associated Document is not fully active, return null.
Let topLevelTraversable be document’s relevant global object’s navigable’s top-level traversable.
If topLevelTraversable.[[DisplayFeaturesOverride]] is non-null, return Viewport’s segments array calculated from topLevelTraversable.[[DisplayFeaturesOverride]].
If there is only a single viewport segment (typically the size of the Viewport) return an array with that single segment.
Otherwise, return the Viewport’s segments array calculated from the hardware features.
The segments property represents an immutable snapshot of the device segments when the value was queried. If the device state changes (rotation, window resize, posture change), the value previously retrieved will be invalid. Developers can listen to Screen Orientation, Window Resize, or Posture changes events to detect device state changes.
If a viewport of 400px by 400px is split horizontally into two side-by-side segments, the
segments
array will have the following content:
[ DOMRect ( 0 , 0 , 400 , 200 ), DOMRect ( 0 , 200 , 400 , 200 )] Appendix A. Changes
This appendix is informative.
Since the 29 March 2016 Working Draftsegments property The segments property poses a challenge to test authors, as exercising this property requires access to specific hardware devices. To address this challenge this document defines [WEBDRIVER2] extension commands that allow users to control how the viewport is split by one or more display features (such as a fold or a hinge between two separate displays).
A display feature override is a struct encapsulating the result of a single display feature. It has a orientation (a string that is either "vertical" or "horizontal"), a mask length (a positive number describing the length of the feature in CSS px), and an offset (which describe the distance from the origin of the viewport in CSS px).
Internal slots To support the extension commands below and their integration with thesegments attribute getter steps, top-level traversables must have the following internal slots: Internal slot Description [[DisplayFeaturesOverride]] List of display feature override that overrides those provided by the hardware, or null. Extensions Commands Set display features HTTP Method URI Template POST /session/{session id}/displayfeatures
This extension command creates a setup that emulates a set of display feature override by taking a list of display features as parameter.
This section is not normative.This section exists because the input format is not trivial. Here is a pseudo IDL example on how a display feature override is defined:
enum OrientationType {
"vertical",
"horizontal"
};
interface DisplayFeature {
readonly attribute OrientationType orientation;
readonly attribute double offset;
readonly attribute double maskLength;
};
Below is an illustration showing the various properties of a display feature:
To create a
[[DisplayFeaturesOverride]]in the
current browsing contextof the
sessionwith ID 23, the
local endwould POST to `/session/23/displayfeatures` with the body:
{
"features": [
{
"orientation": "horizontal",
"offset": 190,
"maskLength": 20
}
]
}
Considering a viewport of 400px by 400px the command will result of a segments property with the following content: [ DOMRect ( 0 , 0 , 400 , 190 ), DOMRect ( 0 , 210 , 400 , 190 )]
Array, return error with WebDriver error code invalid argument.Object, return error with WebDriver error code invalid argument.Number or its value is NaN, +∞, −∞, or negative return error with WebDriver error code invalid argument.Number or its value is NaN, +∞, −∞, or negative return error with WebDriver error code invalid argument.string, return error with WebDriver error code invalid argument.null.This
extension commandremoves the display features override and returns control back to hardware.
null.null.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:
Note, this is an informative note.
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.
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.
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 component 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.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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.
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