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Validator.nu XML Output

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The native XML output format for Validator.nu for integration into content management systems, etc. This format should be able to expose everything there is to expose in Validator.nu results. (Other XML formats may not fit Validator.nu exactly.)

Note: The format has been designed to support streaming generation and consumption.


The elements in this XML vocabulary are in the namespace “http://n.validator.nu/messages/”. This vocabulary reuses elements from the “http://www.w3.org/1999/xhtml” namespace for human-readable messages. The semantics for the elements in the “http://www.w3.org/1999/xhtml” namespace are defined in HTML 5.

  • Perhaps the namespace URI should be a data: URI. If the ns URI does not contain any domain name, it cannot contain a domain name that someone is uncomfortable with. hsivonen 14:24, 18 December 2006 (UTC)

The attributes in this XML vocabulary are not in a namespace. The attribute values defined for this XML vocabulary must not have preceding or trailing white space.

Structure and Semantics

The format consists of an XML 1.0 document that has the element messages as the root element.

The root elements may contain zero or more messages elements (info, error and non-document-error), followed by exactly one verdict element (success, failure or indeterminate), optionally followed by one source element, optionally followed by one parse-tree element.

Message Elements

The element info means an informational message or warning that does not affect the validity of the document being checked. The element 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.)

Locator Attributes

The elements info, error and non-document-error have three optional attributes for indicating the context of the message: uri, line and column. The column attribute must not be present unless the line attribute is present as well.

The uri attribute, if present, must containt the URI (not IRI) of the HTTP 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.)

The line attribute, if present, must contain a string consisting of characters in the range U+0030 DIGIT ZERO to U+0039 DIGIT NINE which when interpreted as a base-ten integer is a positive integer (not zero). This number means the approximate source text line number associated with the message. The first line is 1.

The column attribute, if present, must contain a string consisting of characters in the range U+0030 DIGIT ZERO to U+0039 DIGIT NINE which when interpreted as a base-ten integer is a positive integer (not zero). This number means the approximate source column number associated with the message on the line indicated by the line attribute. The first character on a line is in column 1.

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. Furthermore, implementation may count column numbers in terms of UTF-16 code units instead of characters.

The type Attribute

The info, error and non-document-error element may have an attribute called type for indicating the type of the message in more detail.

The permissible value on the info element 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 on the error element 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 type attribute, the element means a spec violation in general.

Permissible values on the non-document-error element 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 type attribute, the element means a problem external to the document in general.

Children of Message Elements

The info, error and non-document-error elements may contain the following optional elements (in this order): message, elaboration (only if message is present as well) and extract.

The message Element

The message element represents a paragraph of text that is the message stated succinctly in natural language. Permissible element content consists of an interleaving of zero or more text nodes, zero or more a elements in the “http://www.w3.org/1999/xhtml” namespace and zero or more code elements in the “http://www.w3.org/1999/xhtml” namespace. The code elements in the “http://www.w3.org/1999/xhtml” namespace may contain text. The a elements in the “http://www.w3.org/1999/xhtml” namespace may contain an interleaving of zero or more text nodes and zero or more code elements in the “http://www.w3.org/1999/xhtml” namespace. The a elements in the “http://www.w3.org/1999/xhtml” namespace must have the attribute href and may have the attribute title.

The elaboration Element

The elaboration element provides additional human-readable guidance related to the message. The content model of this element is block level content (elements in the “http://www.w3.org/1999/xhtml” namespace) as defined by HTML 5.

The extract Element

The extract element represents and extract of the document source from around the point in source designated for the message. The extract element contains an interleaving zero or more text nodes and exactly one m element. The m element represents a highlighted part of the extract that pinpoints the source position associated with the message. The m element contains the highlighted part of the text. White space is significant in the subtree rooted at extract.

Verdict Elements

An success, failure or indeterminate element represents the end state of the validation (valid, invalid or inability to finish, respectively). These elements take a human-readable message as text content.

The source Element

The source element represents the source of the checked document as decoded to Unicode with XML-unsafe characters replaced with the REPLACEMENT CHARACTER. The element may contain text that is the source. White space is significant in the content.

The element has two optional attributes: type and encoding. The type attribute represents the media type of the input without parameters. The encoding attribute represents the charset media type parameter.

The parse-tree Element

The parse-tree element contains the information items of the parsed infoset that are the children of teh document information item (recursively) encoded as follows:

Comment information items are not represented. Processing instruction information items are represented as element pi with the target in attribute target and data in content. Elements are represented as elements and attributes as attributes, but each namespace ns is substituted with a namespace http://n.validator.nu/?ns=escaped, where escaped is the URI (percent) escaped representation of the UTF-8 representation of ns.

Processing Model

Clients that consume the message format are referred to as processors. They must use a conforming XML 1.0 processor to parse the format.

If the root element is not an element named messages, the document is deemed to be in an unknown format and not processable according to this processing model.

If a processor encounters an element that it doesn’t recognize, it must process the content of the element as if the start tag and the end tag of the element were not there. If the processor encounter character data as a child of the root or a message element element (after applying the rule stated in the previous sentence), it must act as if the character data was not there. If a processor encounters an attribute that it does not recognize, it must ignore the entire attribute. If a processor encounters an attribute that it does recognize but the value of the attribute is not permissible under the previous section, the processor must ignore the entire attribute. If an info, error or non-document-error element does not have a line attribute with a permissible value, a column attribute on the element must be ignored if present.

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

Prior Art

The W3C has defined three XML output formats for the W3C Validator: the SOAP format, the Unicorn format and EARL.

I think there are two problems with the SOAP and Unicorn formats: they are unnecessarily complex and they don’t support streaming output. For example, they require a redundant declaration of the number of errors before the errors themselves (which a client could count on its own if it wants to know the number).

The EARL format assumes that each testable condition has a well-known URI, which does not fit with grammar-based validation and now even with vanilla Schematron.

The W3C Validator also provides simple pass/fail information as HTTP headers, which is nice if you only care about a boolean pass/fail. However, this approach also has the problem the it precludes streaming, because the validation process has to finish before the HTTP headers can be written.

For these reasons, I am not particularly keen on reusing the output formats of the W3C Validator unless it turns out that there are significant network benefits to be reaped from plugging into an existing network of client software. It seems to me that there isn’t a significant network of existing client software.