Filter effects are a way of processing an element’s rendering before it is displayed in the document. Typically, rendering an element via CSS or SVG can conceptually described as if the element, including its children, are drawn into a buffer (such as a raster image) and then that buffer is composited into the elements parent. Filters apply an effect before the compositing stage. Examples of such effects are blurring, changing color intensity and warping the image.
This is Level 2 of the Filter Effects Module.
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This is a public copy of the editors' draft.
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A filter effect is a graphical operation that is applied to an element as it is
drawn into the document. It is an image-based effect, in that it takes zero or
more images as input, a number of parameters specific to the effect, and then
produces an image as output. The output image is either rendered into the
document instead of the original element, used as an input image to another
filter effect, or provided as a CSS image value.
This is Level 2 of the Filter Effects Module. It is currently written as
a "delta", describing any differences from Level 1.
If the value of the backdrop-filter property is none then there is no
filter effect applied. Otherwise, the list of functions are applied in the
The first filter function or filter reference in the list takes
the element’s BackgroundImage as the input image. Subsequent
operations take the output from the previous filter function or filter reference as the input image. filter element
reference functions can specify an alternate input, but still uses the previous
output as its SourceGraphic.
Filter functions must operate in the sRGB color space.
A computed value of other than none results in the creation of a stacking context[CSS21] the same way that CSS opacity does. All the elements descendants are rendered
together as a group with the filter effect applied to the group as a whole.
The result of the backdrop-filter is rendered before the other painting
operations for the element. That is, behind any background of the element.
If the filter functions would have produced a result that extended beyond
the bounds of the input, it is clipped to the original bounds.
The use of this property may have an adverse effect on
on performance, especially when applied to a number of elements, or
a large area of the page. Authors should be careful to use it on a
minimal amount of content.
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",
This is an example of an informative example.
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.
Conformance to this specification
is defined for three conformance classes:
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
Implementations of Unstable and Proprietary Features
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