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<h2 id="conformance" class="no-ref no-num">
Conformance</h2>

<h3 id="document-conventions">
Document conventions</h3>

    <p>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.

    <p>All of the text of this specification is normative except sections
    explicitly marked as non-normative, examples, and notes. [[!RFC2119]]

    <p>Examples in this specification are introduced with the words “for example”
    or are set apart from the normative text with <code>class="example"</code>,
    like this:

    <div class="example">
        <p>This is an example of an informative example.
    </div>

    <p>Informative notes begin with the word “Note” and are set apart from the
    normative text with <code>class="note"</code>, like this:

    <p class="note">Note, this is an informative note.

    <p>Advisements are normative sections styled to evoke special attention and are
    set apart from other normative text with <code>&lt;strong class="advisement"></code>, like
    this:

    <strong class="advisement">
        UAs MUST provide an accessible alternative.
    </strong>

<h3 id="conform-classes">
Conformance classes</h3>

    <p>Conformance to this specification
    is defined for three conformance classes:
    <dl>
        <dt>style sheet
            <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
            style sheet</a>.
        <dt>renderer
            <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a>
            that interprets the semantics of a style sheet and renders
            documents that use them.
        <dt>authoring tool
            <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a>
            that writes a style sheet.
    </dl>

    <p>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.

    <p>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.)

    <p>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.

<h3 id="conform-responsible">
Requirements for Responsible Implementation of CSS</h3>

    <p>The following sections define several conformance requirements
        for implementing CSS responsibly,
        in a way that promotes interoperability in the present and future.

<h4 id="conform-partial">
Partial Implementations</h4>

    <p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
        (and <a href="http://www.w3.org/TR/CSS2/conform.html#ignore">ignore as appropriate</a>)
        any at-rules, properties, property values, keywords, and other syntactic constructs
        for which they have no usable level of support</strong>.
        In particular, user agents <em>must not</em> selectively ignore
        unsupported property 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.

<h4 id="conform-future-proofing">
Implementations of Unstable and Proprietary Features</h4>

    <p>To avoid clashes with future stable CSS features,
        the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS.

<h4 id="conform-testing">
Implementations of CR-level Features</h4>

    <p>Once a specification reaches the Candidate Recommendation stage,
        implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> implementation
        of any CR-level feature they can demonstrate
        to be correctly implemented according to spec,
        and should avoid exposing a prefixed variant of that feature.

    <p>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.

    <p>Further information on submitting testcases and implementation reports
        can be found from on the CSS Working Group’s website at <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
        Questions should be directed to the <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a> mailing list.

<h3 id="cr-exit-criteria" class="no-ref">
CR exit criteria</h3>

    <p>
    For this specification to be advanced to Proposed Recommendation,
    there must be at least two independent, interoperable implementations
    of each feature. Each feature may be implemented by a different set of
    products, there is no requirement that all features be implemented by
    a single product. For the purposes of this criterion, we define the
    following terms:

    <dl>
        <dt>independent <dd>each implementation must be developed by a
        different party and cannot share, reuse, or derive from code
        used by another qualifying implementation. Sections of code that
        have no bearing on the implementation of this specification are
        exempt from this requirement.

        <dt>interoperable <dd>passing the respective test case(s) in the
        official CSS test suite, or, if the implementation is not a Web
        browser, an equivalent test. Every relevant test in the test
        suite should have an equivalent test created if such a user
        agent (UA) is to be used to claim interoperability. In addition
        if such a UA is to be used to claim interoperability, then there
        must one or more additional UAs which can also pass those
        equivalent tests in the same way for the purpose of
        interoperability. The equivalent tests must be made publicly
        available for the purposes of peer review.

        <dt>implementation <dd>a user agent which:

        <ol class=inline>
            <li>implements the specification.

            <li>is available to the general public. The implementation may
            be a shipping product or other publicly available version
            (i.e., beta version, preview release, or "nightly build").
            Non-shipping product releases must have implemented the
            feature(s) for a period of at least one month in order to
            demonstrate stability.

            <li>is not experimental (i.e., a version specifically designed
            to pass the test suite and is not intended for normal usage
            going forward).
        </ol>
    </dl>

    <p>The specification will remain Candidate Recommendation for at least
    six months.

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