This page describe the checker’s JSON output format.

Italicized words, such as object, refer to JSON data types. “The "foo" datatype” refers to an object of type datatype that is the value associated with the key "foo" in the parent object.

The Internet media type for this format is application/json. (Unless the callback extension is used, in which case the media type is application/javascript.)

The root object is a JSON object. It has one mandatory key, "messages", and three optional keys, "url", "source", and "language".

The values for these keys are described below.

This array is an ordered collection of zero or more message objects.

A message object has one mandatory key, "type", and seven optional keys, "subtype", "message", "extract", "offset", "url", "line" and "column".

The "type" string denotes the general class of the message. The permissible values are "info", "error" and "non-document-error".

"info" means an informational message or warning that does not affect the validity of the document being checked. "error" signifies a problem that causes the validation/checking to fail. "non-document-error" signifies an error that causes the checking to end in an indeterminate state because the document being validated could not be examined to the end. Examples of such errors include broken schemas, bugs in the validator and IO errors. (Note that when a schema has parse errors, they are first reported as errors and then a catch-all non-document-error is also emitted.)

The permissible value with "type":"info" is "warning", which means that the message seeks to warn about the user of a formally conforming but in some way questionable issue. Otherwise, the message is taken to generally informative.

The permissible value with "type":"error" is "fatal", which means that the error is an XML well-formedness error or, in the case of HTML, a condition that the implementor has opted to treat analogously to XML well-formedness errors (e.g. due to usability or performance considerations). Further errors are suppressed after a fatal error. In the absence of the "subtype" key, a "type":"error" message means a spec violation in general.

Permissible values with "type":"non-document-error" are: "io" (signifies an input/output error), "schema" (indicates that initializing a schema-based validator failed) and "internal" (indicates that the validator/checker found an error bug in itself, ran out of memory, etc., but was still able to emit a message). In the absence of the "subtype" key, a "type":"non-document-error" message means a problem external to the document in general.

The "message" string represents a paragraph of text (suitable for rendering to the user as plain text without further processing) that is the message stated succinctly in natural language.

The "extract" string represents an extract of the document source from around the point in source designated for the message by the "line" and "column" numbers.

The "offset" number is an UTF-16 code unit index into the "extract" string. The index identifies the same UTF-16 code unit in the extract that the "line" and "column" numbers identify in the full source. The first code unit has the index 0.

The "url" string, if present, must contain the URI (not IRI) of the resource with which the message is associated or the literal string “data:…” (the last character is U+2026) to signify that the message is associated with a data URI resource but the exact URI has been omitted. (If a client application wishes to show IRIs to human users, it is up to the client application to convert the URI into an IRI.)

If the "url" string is absent on the message element but present on the root element, the message is considered to be associated with the resource designated by the attribute on the root element.

The "firstLine", "firstColumn", "lastLine" and "lastColumn" numbers indicate a range of source code associated with the message. The line and column numbers are one-based. The first line is line 1. The first column is column 1. Columns are counted by UTF-16 code units. A line break is considered to occupy the last column on the line it terminates.

The source lines and columns are approximate. For example, if a message is related to an attribute, the line and column may point to the first character if the start tag, the character after the start tag or to the attribute inside the tag depending on implementation. If a message is related to character data, the line and column may be inaccurate within a run of text e.g. due to buffering.

The "lastLine" number indicates the last line (inclusive) onto which the source range associated with the message falls.

The "firstLine" number indicates the first line onto which the source range associated with the message falls. If the attribute is missing, it is assumed to have the same value as "lastLine".

The "lastColumn" number indicates the last column (inclusive) onto which the source range associated with the message falls on the last line onto which is falls.

The "firstColumn" number indicates the first column onto which the source range associated with the message falls on the first line onto which is falls.

The "url" string, if present, must contain the URI (not IRI) of the document being checked or the literal string “data:…” (the last character is U+2026) to signify that the message is associated with a data URI resource but the exact URI has been omitted. (If a client application wishes to show IRIs to human users, it is up to the client application to convert the URI into an IRI.)

A "source" object has one mandatory key, "code", and two optional keys, "type" and "encoding".

The "code" string represents the source of the checked document as decoded to Unicode lone surrogates replaced with the REPLACEMENT CHARACTER and with line breaks replaced with U+00A0 LINE FEED.

The "type" string represents the media type of the input without parameters.

The "encoding" string represents the charset media type parameter of the input.

The "language" string represents the detected language of the checked document. Its value is a BCP 47 language tag. The absence of the "language" string indicates that the language of the checked document could not be determined with confidence.

{
    "url": "http://example.org/",
    "messages": [
        {
            "type"   : "info",
            "subtype": "warning",
            "lastLine"   : 20,
            "lastColumn" : 15,
            "url"    : "http://example.com/",
            "message": "Trailing slash for void element",
            "extract": "<br/>",
            "hiliteStart" : 3,
            "hiliteLength" : 1
        },
        {
            "type"   : "error",
            "subtype": "fatal",
            "lastLine"   : 42,
            "lastColumn" : 17,
            "url"    : "http://example.com/",
            "message": "Missing end tag for the “foo” element"
        }
    ],
    "source": {
        "code"    : "...",
        "type"    : "text/html",
        "encoding": "UTF-8"
    },
    "language": "en"
}

Clients that consume the message format are referred to as processors. They must use a parser conforming to RFC 4627 to parse the format.

If the root is not an object with the key "messages", the JSON text is deemed to be in an unknown format and not processable according to this processing model.

If the processor encounters a key–value pair in an object with a known key and an unknown value where a value enumerated in this specification is expected, the processor must ignore the key–value pair. If a processor encounters an object that is missing a required key (possibly because it was ignored under the previous rule), the processor must ignore the entire object. If a message object does not have a "line" number with a permissible value, a "column" number on the object must be ignored if present.

Processors must process the items in a way that is consistent with the semantics of the items.

The outcome of the validation process may be success, failure or indeterminate.

1. If there are one or more non-document-error messages, the outcome is indeterminate.
2. Else if there are one or more error messages, the outcome is failure.
3. Else the outcome is success.

The format described here may optionally be wrapped in a JavaScript function call.

See also: Web Service Interface