CSS Media Queries defines a mechanism that allows adapting the layout and behavior of a Web page based on some of the characteristics of the device, most notably based on the screen resolution.
The viewport-relative CSS units
vmax represent a percentage of the current viewport dimensions and let developers design layouts that automatically adapt to viewport dimensions changes.
The Responsive Images Community Group (RICG) developed an extension to HTML, known as the
picture element, that allows authors define what image to load depending on device capabilities and/or other media features.
As a complementary approach, the
srcset attribute, specified by the WHATWG and also published as an extension to HTML, let Web developers define the various device pixel ratios of an image, letting the browser pick the best choice for the pixel density of the screen.
SVG, which lets define images that can be scaled up and down without any loss of quality, is another critical tool to the development of Web applications that adapt to the resolution of the underlying device.
Web components are a set of features that together allow applications to encapsulate their logic in re-usable components. Primary technologies used to create Web components:
<template>element allows to declare fragments of HTML that can be cloned and inserted in the document by script.
- Custom elements provides a way for authors to build their own fully-featured DOM elements.
- The shadow DOM allows the web browser to render DOM elements without putting them into the main DOM document tree.
Technologies in progress
CSS Device Adaptation defines a set of CSS directives to define the size on which this layout should be based, relatively to the size of the underlying device — specifying what has been implemented using the
<meta name="viewport"> element so far.
CSS Mobile Text Size Adjustment lets text adapt to zoomed parts of a page.
CSS Media Queries Level 4 and 5 add several relevant hooks to adapt the user interface in mobile contexts, including:
- The availability and type of a pointing device through the
pointerfeature (and its companion
- The ability to hover elements through the
- The ability to react to ambient luminosity changes or to follow users' preferences for a light or dark theme through the
- The need to reduce the amount of animations in the page, e.g. to save battery, through the
- The ability to react to script-defined variables, making it easier to cater for more complex adaptation rules in the overall logic of the application.
Specific CSS technologies can target and style Web components:
- CSS Scoping lets authors define default styles for custom elements.
- CSS Shadow Parts enables flagging elements in a shadow tree and styling them in the outside page, thus providing an easy-to-use mechanism to customize styles of custom elements as needed.
- Constructable Stylesheet Objects provides an API for creating stylesheet objects from script without needing style elements, making adding styles to web components more convenient and performant.
As described in Use Cases and Requirements for Element Queries, developers need to control the layout of an individual element relative to the size of its container to create complex responsive applications that adapt to a wide variety of devices and contexts. Media queries and the
resize event on the
window element only allow developers to adjust the layout of a component relative to the viewport size. Resize Observer defines an interface for observing changes to an element's size and responding to these changes, allowing to design responsive Web components that can be used across applications and devices.
A common approach to content adaptation on mobile devices is to have the server deliver the content that fits the user's device right away. This approach avoids sending content to the client that the client does not need, preserving network bandwidth, and it avoids running the adaptation logic on the client itself, preserving CPU and memory. Server-side adaptation used to rely on sniffing the
User-Agent string and mapping it to a database of devices to extract relevant capabilities. This mechanism is error-prone and requires continuous maintenance of the database of devices. Through the HTTP Client Hints mechanism, developed by the IETF HTTP Working Group, clients and servers may now opt-in to exchange relevant capabilities for efficient content adaptation. The Web Performance Working Group has started to work on a Device Memory specification that extends these hints to surface the memory available on the device. The specification also defines an API to surface that information to the Web application running on the client.
Media Capture Capabilities
The Media Capture and Streams API exposes some specific information on capabilities of camera and microphones to make it possible to take advantage of the large variety of media capturing devices provided on mobile phones.
Most mobile devices feature on-screen keyboards for user input. They also commonly let the user pinch-zoom into a page. Both of these features preserve the layout viewport (what the web page uses when laying out its user interface), so as not to clutter the screen with
position: fixed elements. The Visual Viewport API provides developers with a way to query and interact with the properties of the visual viewport (the box on the page that the user can currently see), and to be notified when these properties change.
The User-Agent Client Hints proposal defines a set of client hints that provide developers with the ability to perform agent-based content negotiation when necessary, while avoiding the historical baggage and passive fingerprinting surface exposed by the
User-Agent header, which was at the basis of server-side adaptation.
|Feature||Specification / Group||Implementation intents|
|JS-based adaptation||Visual Viewport API|
Web Platform Incubator Community Group
|Server-side adaptation||User-Agent Client Hints|
Web Platform Incubator Community Group