<image> - CSS: Cascading Style Sheets 编辑

The <image> CSS data type represents a two-dimensional image.

Syntax

The <image> data type can be represented with any of the following:

• An image denoted by the <url>() data type
• A <gradient> data type
• A part of the webpage, defined by the element() function
• An image, image fragment or solid patch of color, defined by the image()() function
• A blending of two or more images defined by the cross-fade() function.
• A selection of images chosen based on resolution defined by the image-set() function.

Description

CSS can handle the following kinds of images:

• Images with intrinsic dimensions (a natural size), like a JPEG, PNG, or other raster format.
• Images with multiple intrinsic dimensions, existing in multiple versions inside a single file, like some .ico formats. (In this case, the intrinsic dimensions will be those of the image largest in area and the aspect ratio most similar to the containing box.)
• Images with no intrinsic dimensions but with an intrinsic aspect ratio between its width and height, like an SVG or other vector format.
• Images with neither intrinsic dimensions, nor an intrinsic aspect ratio, like a CSS gradient.

CSS determines an object's concrete size using (1) its intrinsic dimensions; (2) its specified size, defined by CSS properties like width, height, or background-size; and (3) its default size, determined by the kind of property the image is used with:

The concrete object size is calculated using the following algorithm:

• If the specified size defines both the width and the height, these values are used as the concrete object size.
• If the specified size defines only the width or only the height, the missing value is determined using the intrinsic ratio, if there is any, the intrinsic dimensions if the specified value matches, or the default object size for that missing value.
• If the specified size defines neither the width nor the height, the concrete object size is calculated so that it matches the intrinsic aspect ratio of the image but without exceeding the default object size in any dimension. If the image has no intrinsic aspect ratio, the intrinsic aspect ratio of the object it applies to is used; if this object has none, the missing width or height are taken from the default object size.

Accessibility concerns

Browsers do not provide any special information on background images to assistive technology. This is important primarily for screen readers, as a screen reader will not announce its presence and therefore convey nothing to its users. If the image contains information critical to understanding the page's overall purpose, it is better to describe it semantically in the document.

• MDN Understanding WCAG, Guideline 1.1 explanations
• Understanding Success Criterion 1.1.1 | W3C Understanding WCAG 2.0

Examples

Valid images

url(test.jpg)               /* A <url>, as long as test.jpg is an actual image */
linear-gradient(blue, red)  /* A <gradient> */
element(#realid)            /* A part of the webpage, referenced with the element() function,
                               if "realid" is an existing ID on the page */
image(ltr 'arrow.png#xywh=0,0,16,16', red)
                            /* A section 16x16 section of <url>, starting from the top, left of the original
                               image as long as arrow.png is a supported image, otherwise a solid
                               red swatch. If language is rtl, the image will be horizontally flipped. */
cross-fade(20% url(twenty.png), url(eighty.png))
                            /* cross faded images, with twenty being 20% opaque
                               and eighty being 80% opaque. */
image-set('test.jpg' 1x, 'test-2x.jpg' 2x)
                            /* a selection of images with varying resolutions */

Invalid images

nourl.jpg            /* An image file must be defined using the url() function. */
url(report.pdf)      /* A file pointed to by the url() function must be an image. */
element(#fakeid)     /* An element ID must be an existing ID on the page. */
image(z.jpg#xy=0,0)  /* The spatial fragment must be written in the format of xywh=#,#,#,# */
image-set('cat.jpg' 1x, 'dog.jpg' 1x) /* every image in an image set must have a different resolutions */

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• <gradient>
• element()
• image()
• image-set()
• cross-fade()