XML-in-the-browser interoperability tests

C. M. Sperberg-McQueen

Black Mesa Technologies LLC

Copyright © 2012 by the author

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XML-in-the-browser interoperability tests

Balisage: The Markup Conference 2012
August 7 - 10, 2012

Most current Web browsers support the retrieval of XML from Web hosts, the application of CSS and XSLT stylesheets to that XML, and the display of XML documents in the browser. But publication of XML on the Web is not always a straightforward task. Does the browser validate the document? If the document is invalid, will the browser display it anyway or display only an error message? Will the browser expand entity references? Will it process the DTD? Will it process the internal DTD subset but not the external subset? Will it refuse under certain circumstances to fetch the appropriate stylesheet or DTD files?

These are not questions of specification conformance: the XML specification does not impose answers to these or similar questions as requirements of XML conformance. But they and others like them are important questions for anyone who would like to publish XML documents on the Web in a way that works with a broad variety of current browsers.

The XiBIT project aims to help. The name XiBIT (pronounced like the English word exhibit) stands for XML-in-the-browser interoperability tests; the goal of the project is to investigate and document the behavior of current Web browsers in the processing and display of XML. It is hoped that the results of XiBIT will be helpful to anyone who wishes to use XML on the Web, by tabulating usages that work consistently across browsers (or conversely by identifying usages or constructs which lead to different behavior in different browsers).

This paper describes the current state and plans of XiBIT. An introductory section includes some remarks on related work, a discussion of the difference between interoperability testing and conformance testing, and a brief discussion of some relevant historical background. This is followed by a description of the project, covering aims, expected work products, and work methods, and by some sample test cases and results. A list of tasks for future work and a brief conclusion complete the paper.


Related work

Much of the obviously related work consists of confomance tests.

Conformance and other test suites already exist for many of the technologies involved in displaying XML on the Web.

Shortly after the XML specification was published, the Organization for Organization for the Advancement of Structured Information Standards (OASIS) chartered a technical committee to develop an XML conformance test suite on the basis of a earlier efforts by XML implementors and others; the committee's report mentions contributions from James Clark, Fuji Xerox, Sun Microsystems, IBM, and a joint effort by OASIS and the U.S. National Institute of Standards and Technology (NIST) [OASIS 2001]. After a revision of the test suite for XML 1.0 Second Edition, the Oasis work was transferred to the XML Core working group of W3C (the World Wide Web Consortium); the current version of the test suite, released in 2008, covers XML 1.0 Fifth Edition, XML 1.1, and the corresponding editions of the Namespaces in XML recommendation [W3C 2008[?]].

A test suite for XSLT 1.0 was developed by the W3C XSL working group and used to provide evidence of implementation (which is required by the W3C process before specifications are published as W3C Recommendations), but it is accessible to W3C members only and has not been made publicly available. (The same is true of the W3C's test suite for XSLT 2.0.)

Another strain of related endeavor includes the work done by the W3C's HTML 5 and Web Applications working groups, which has done a great deal to clarify the behavior of existing server and browser software. XiBIT differs from that work partly in its exclusive focus on XML rather than HTML, partly in its current focus on serving human-readable documents directly and styling them using XSLT rather than on serving XML data for use by Javascript code running in the browser and manipulating it via the DOM; and partly in that XiBIT is aimed solely at documentation of existing practice and not at the development of any new specifications.

As the XiBIT project develops, it may integrate or adapt material both from conformance test suites for the relevant specifications and from test collections like those used by the HTML5 and Web Applications working groups to document existing practice.

A third line of related work may be found in the investigations of Alex Milowski over the last few years into the possibilities for augmenting and improving XML support in Web browsers [Milowski 2009], [Milowski 2011]. Milowski's work overlaps with XiBIT in part; at least, like XiBIT, he records some interoperability issues with recent and current browsers. He raises those issues, however, as a way of making rhetorical points about the need for more aggressive action by the XML community; XiBIT's aims are less hortatory and more systematic. Milowski is interested in possiblities for extending Web browsers and improving their XML support; XiBIT restricts itself for the moment to recording their status quo behavior.

Interoperability testing vs. conformance testing

The collection of tests developed by the XiBIT project differs from conformance test suites in several ways.

First and foremost, XiBIT focuses on interoperability testing or documentation of behavior, not conformance.

Note that strictly speaking, interoperability denotes the ability of client- and server-side implementations of network protocols to communicate successfully with each other. Its application to specifications like XML and XSLT, which define data formats or operations on data which can be and usually are performed by individual implementations acting alone, requires that the term be given a different meaning. In the implementation of communications protocols, interoperability helps to establish both that the specification is clearly enough defined to be implemented consistently (thus providing a quality check on the specification itself) and also that users of the technology can switch from one implementation to another without (excessive) loss of functionality. As [ISO/IEC 2381-01] defines the term, interoperability is The capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units. In the case of single-processor specifications like XML and XSLT, it is consistency among independent implementations that helps establish these properties.[1]

XiBIT's focus on interoperability testing rather than conformance testing means it differs from conformance tests in several ways:

  • Serious conformance tests need thorough coverage of the specification(s) being implemented; they often include thousands or tens of thousands of tests. At least in its initial versions, XiBIT does not aim at that kind of coverage.

  • Conformance tests have right and wrong answers; XiBIT tests do not have right or wrong answers. They seek only to find out what processors actually do under the tested circumstances. (If the behavior in question is clearly governed by a particular specification which clearly prescribes a particular behavior, then the demands of the specification are of some interest to most users interested in open standards, as well as to most implementors. But strictly speaking the relation of XiBIT tests to conformance is a side issue.)

  • Almost all specifications leave some aspects of behavior unspecified, to avoid over-constraining implementations, to leave some conceptual space clear for implementations to compete against each other, and/or to leave flexibility for implementations to adapt to their expected users and operating environments.

    Conformance tests should not, and typically do not, test behavior in these areas, which stands to reason: testing behaviors not constrained by a specification cannot in principle provide information about the conformance or non-conformance of the software under test. For interoperability as defined here, however, behaviors not constrained by the controlling specifications can be just as interesting and relevant as behaviors that are constrained. Indeed, whenever most implementations are reasonably conformant, interoperability testing is likely to focus all the more on behaviors which are not constrained by conformance to the specifications, because that is where variation among implementations will be found. And for the same reasons, when not all implementations are reasonably conformant, interoperability testing will in practice need to devote more attention to areas where conformance is weak in some implementations.

  • Those who want to publish XML on the web will need to attend to browser and server behavior which is not in fact governed by any de jure or de facto standard, or sometimes by any written specification at all.

Historical background

When in the early 1990s the World Wide Web exploded in popularity, it placed the SGML community in a delicate, slightly awkward position. HTML was manifestly an application of descriptive markup: its syntax was the reference concrete syntax of SGML, its element set clearly modeled on (and in part identical with) the starter set of IBM DCF, Waterloo Script, and Annexe E.1 of ISO 8879. The HTML spec paid lip service to the ideas of descriptive markup. So the success of HTML and the World Wide Web could, with a little effort, be interpreted as reflecting well on descriptive markup and SGML. But at the same time, the Web had characteristics which many SGML users found troubling, even distasteful. Web browsers were limited to a single vocabulary which few SGML users much liked, they did not follow the standard rules of SGML parsing, and the HTML specification prescribed behavior in the presence of invalid input which was foreign to SGML practice. Some SGML users found it hard to like an application which deviated so markedly from what they regarded as good practice, and some found it frustrating that the Web found wider uptake than other SGML applications they regarded as better designed.

It did not take very long for some in the SGML community to conceive of a World Wide Web that accommodated their SGML data without requiring a down-translation into HTML. Through a history that does not need to be rehearsed in detail here, this idea led to the formation of a working group at the World Wide Web Consortium (W3C) chartered to formulate a version of SGML that would be suitable for use on the Web. The end result of this SGML on the Web Working Group was a subset of SGML called (so as to differentiate it from any markup system with a fixed single vocabulary, like HTML) the Extensible Markup Language, XML.

One stated goal of the SGML on the Web working group was to make it possible to deploy XML documents on the Web; this intended application led in turn to the development of some other specifications (e.g. for the XML stylesheet instruction W3C 2010) and affected the direction taken by others: Cascading Stylesheets (CSS) was reformulated to enable CSS to work with any vocabulary, not only with HTML, and XSLT 1.0 was designed to be lightweight enough to be implementable in a Web browser.

If one of the goals of the work that led to XML was to make it possible to deploy XML documents on the Web, then it makes sense to ask how far that goal has been achieved, and under what circumstances it's possible. That is one of the rationales for XiBIT.

Project description


The goal of XiBIT is to investigate and document the behavior of existing Web browsers in the processing and display of XML.

Expected work products

XiBIT will generate several work products:

  • a set of tests, available from a Web server

  • a tabulation of browser behavior on those tests

  • a public interface to allow volunteers to submit data recording behavior of specific browsers in specific environments

  • prose documentation of XML browser behaviors

  • a diagnostic tool to read XML documents and issue warnings of potential interoperability issues

Some of these exist now (samples and screen shots below); others will be developed as the project continues. All materials developed by XiBIT will be made publicly available under the GNU Public License at the project's Web site, http://xibit.blackmesatech.com/.


Identifying axes of variation

XiBIT tests are written to explore browser behaviors that may affect interoperability.

Each set of tests explores one or more dimensions along which XML documents can vary, so as to see how that variation affects browser behavior. The set of dimensions explored by a test set may be referred to as a space of variation.

The dimensions or axes of variation to be tested are found in several ways.

  • The project will perform a systematic search of the Web for documents describing pitfalls of XML publication and techniques for avoiding them.

  • Even without a survey of existing literature on the subject, some axes suggest themselves to introspection based on practical experience serving XML documents on the Web. For example: browser behavior has in the past been affected by the validity or invalidity of the XML document, by the location of DTD files and stylesheets, by the presence or absence of document-type declarations in XSLT stylesheets, etc., etc. Some failures of interoperability among browser-based XSLT implementations reflect conformance issues in the browsers' implementations of XSLT: Mozilla-based browsers do not support the namespace axis, several browsers have (different) bugs in their handling of the XSLT unparsed-entity-uri() function, and so on.

  • Once the first version of XiBIT is made public, the project site will solicit input from the public. Any problem that arises for anyone in publishing XML documents on the Web may point to some relevant variations in browser behavior, so any war story about things going wrong in XML publishing may help XiBIT improve its coverage.

  • Other areas of variation can be identified by systematic examination of the relevant specifications: XML, Namespaces in XML, XSLT 1.0, XML Base, Associating Style Sheets with XML documents, Cross-origin resource sharing, the Document Object Model, etc. For the reasons described earlier, particular attention will be paid to areas left unconstrained by the relevant specifications.

  • Finally, the project will (time permitting) review relevant conformance test suites and the work of relevant working groups; many questions addressed for HTML documents by the HTML5 work may also arise with respect to XML documents. Because of the way current browsers are constructed, they may behave differently for HTML and XML documents.

Size of the space of variation

Each axis of variation will have at least two possible values and may have more. For example, processor variation in the handling of DTDs may be affected by:

  • whether the document is valid or invalid;

  • whether the document type definition has an internal subset (declarations in the document entity itself), an external subset (declarations in other resources), or both;

  • whether any external document type definition resources are pointed to in a way that violates the browser's same-origin policy; in different contexts, browsers may behave differently for relative references that point downward in the virtual directory structure of URIs, relative references that point upward in that structure, relative references that appear to point upward (i.e. they begin with ..) but in fact do not (they re-descend to the original location), absolute references that refer to the same host, absolute references that refer to a different host, and references which use a different URI scheme, different port number, etc. For purposes of the examples below, it is supposed that there are six relevant variants for the form of the URI.

The retrieval of stylesheets may be affected by the same variations in location of the stylesheet.

In principle, these variables can interact in a browser, so a complete survey will require testing all possible combinations. In simple cases, the number of test cases required is the product of the number of possible values on each dimension, and the set of test cases to be constructed for a given space of variation is the Cartesian product of the dimensions. In most cases, however, not all combinations of values are logically possible, so the actual number of cases will be lower (and may be difficult to calculate a priori). An example may serve to illustrate the point. In the case of XML documents with and without a DTD and with and without a single CSS or XSLT stylesheet, the exhaustive approach would require all combinations of validity, DTD composition, external DTD location, CSS location, and XSLT location, for 2 (valid vs. invalid) × 2 (external subset with or without and internal subset) × 6 (variations in relative and absolute URIs for the DTD) × 6 (variations in relative and absolute URIs for the stylesheet) × 2 (CSS vs. XSLT stylesheet) or 288 cases for documents with an external DTD and one stylesheet, plus another 36 (= 2 forms of validity × 2 forms of DTD structure × 6 forms of URI) for those with an external DTD and no stylesheet, plus 24 (= 2 forms of validity × 6 forms of URI × 2 kinds of stylesheet) for those with no DTD and one stylesheet, plus 1 for those with neither DTD nor stylesheet, for a total of 349 tests.

In practice, many of the theoretically possible interactions among variables do not arise. If a browser shows the user an error message instead of the document, when the document is invalid and has no stylesheet, it is unlikely though not impossible that the browser will show the invalid document to the user if only the document has an XSLT or CSS stylesheet. To reduce the overall number of tests, then, and make it easier to examine the behavior of a browser on all tests, the XiBIT project works on the assumption that different axes of variation should be assumed to be independent unless evidence arises that suggests that they interact. Browser behavior with respect to location of DTDs and browser behavior with respect to location of CSS and XSLT stylesheets are tested independently, with 39 tests, rather than 349: 24 documents with external DTDs (= 2 forms of validity × 6 forms of URI × 2 for the presence or absence of an internal subset), 2 with internal DTDs only (valid and invalid), 1 with no DTD at all and no stylesheet, plus 6 tests each for various forms of URI pointing to an XSLT or CSS stylesheet.[2]

Generating tests

For a given space of variation, XiBIT tests are generated automatically using a two-step process: first a program generates an XML description of the set of test cases, and then a second program generates from this description both the individual files needed for the test cases and a list of the tests in the form of a catalog. (In the current version of XiBIT, the first program is written in XQuery and the second in XSLT.)

Samples of the test-case generator code and test cases are given below.

Testing browsers

Test cases take the form of XML documents which may have references to a DTD and/or stylesheets. The simplest way to run a given test case in a given browser is just to dereference that test case using that browser, and see what the browser does with it. Possible behaviors include styling the document with the given stylesheet, displaying the document in tagged form (as most browsers now do with data recognized as XML that has no XML stylesheet processing instruction), treating the document as if it were HTML (i.e. ignoring all tags unless they have the same names as HTML elements and displaying the #PCDATA content of the document), displaying a blank screen, and displaying an error message.

For this kind of testing, the simplest presentation of a set of test cases is just a directory listing, as shown in figure 1.

Figure 1

png image ../../../vol8/graphics/Sperberg-McQueen01/Sperberg-McQueen01-001.png

If a set of tests has more than a handful of test cases, however, dereferencing each test case individually is tedious (one click to navigate to the test case, one click to navigate back or to close the separate tab or window), and the temptation to skip a few tests rapidly becomes overpowering.

For some kinds of software and test suite, it's possible to build a test harness to run all the tests automatically and record the results, comparing them automatically to the expected result of the test. For interactive software, however, the result of the test often consists not in output that could be written to a file and checked automatically, but in this or that rendering of the XML document on the screen; a human is required to evaluate the result. (The browser window could in principle be captured in an image file and then analysed automatically, but that kind of automated image analysis goes far beyond the resources available to XiBIT.) So there is a limit to the possible automation.

It's possible, however, to use the HTML iframe and object elements to embed test cases in another document; within some limits, this makes it possible to inspect the results of many tests in a single page, as shown in figure 2.[3]

Figure 2

png image ../../../vol8/graphics/Sperberg-McQueen01/Sperberg-McQueen01-002.png

Tabulation of results

The raw results will be stored in an XML database and made available via a search interface so that the set of results can be filtered by operating system, browser, and test case. Details are not yet worked out.

Some sample tests

The more complicated a test case, the more different things there are that might conceivably affect the way it is treated by a browser. To simplify interpretation of the results, therefore, XiBIT tries where possible to keep the test cases as simple as possible.

To test the effect of DTD location and validity, for example, test cases of the following form are used. The external DTD has a single declaration:

<!ELEMENT valid-document (#PCDATA) >
Valid documents take the following form; the form of the system identifier is the only thing that varies.
<!DOCTYPE valid-document SYSTEM "lib/wabash.dtd">
  This test document is part of XiBIT:
  XML in the browser interface tests.
Invalid documents differ only in the element's generic identifier:
<!DOCTYPE valid-document SYSTEM "lib/wabash.dtd">
  This test document is part of XiBIT:
  XML in the browser interface tests.

Tests involving stylesheets are similarly simple in structure and have no DTD (on the assumption described above that DTD processing and stylesheet processing can be described independently, unless we encounter evidence that they interact). The following test applies a stylesheet whose URI is a downward-pointing relative reference:

<?xml-stylesheet type="text/xsl" href="lib/down.xsl" ?>
  This test document is part of XiBIT:
  XML in the browser interface tests.

As mentioned earlier, the tests are produced systematically by an XQuery module, which defines a function for each space of variation. An example may illustrate the construction of these functions.

For testing the effect of location and stylesheet language on stylesheet processing, the relevant function begins by defining the relevant axes of variation: XSL vs. CSS,

(: define yesno so we can iterate over it conveniently :)
    let $yesno := (true(), false())
(: Define the major axes so we can iterate over them :)
(: Axis 1:  XSL or CSS? :)
    for $lang in ('xsl', 'css')
(: Axis 2:  location of the stylesheet:  
            absolute or relative?
            Same host or different?  same scheme?
            Downward path? Upward? Deceptive path?
        (: Is the URI absolute or relative? :)
        $uritype in ('relative', 'absolute'),
        $reltype in ('', 'down', 'up', 'deceptive'),
        $abstype in ('', 'abs', 'diffhost', 'diffscheme')
These axes are not orthogonal; a where clause is used to filter out tuples[4] whose values form a nonsensical combination (like a relative URI reference pointing to a different host).[5]
(: Specify consistency constraints :)
  where (: only relative URIs have relative types :) 
        ($uritype eq 'relative' or not($reltype))
    and (: only absolute URIs can have abstype :)
        ($uritype eq 'absolute' or not($abstype))
    and (: one or the other, please :)
        ( ($reltype) or ($abstype) or not($uritype))
For each tuple of bindings satisfying the where clause, the function then returns a test case description element. (It would be possible to generate the test case directly, but it proved simpler to separate the tasks of formulating a description and actually writing out the test case.)
 return element test-case {
          attribute id { 
            concat($lang, '.',
              if ($uritype) then
                 concat(substring($uritype,1,3), '.',
                        $reltype, $abstype)
              else '')	
          element stylesheet-link {
             attribute type {$lang},
             if ($uritype eq 'relative') then
                attribute uri {$reltype}
                attribute uri {$abstype}

The query produces elements like the following two, which describe test cases which call an XSLT stylesheet with two forms of relative URI:

  <test-case id="xsl.rel.down">
    <trace lang="xsl" uri="relative" reltype="down"/>
    <stylesheet-link type="xsl" uri="down"/>
  <test-case id="xsl.rel.up">
    <trace lang="xsl" uri="relative" reltype="up"/>
    <stylesheet-link type="xsl" uri="up"/>
In the next step, these descriptions are processed to produce the actual test cases; the first of these two descriptions produces the XSLT test case shown above.

For testing the effect of DTD location and validity on processing, the relevant function has the same structure, though there are more axes of variation: whether an internal subset exists, whether an external subset exists, the nature of the URI for an external subset, and validity:

(: define yesno so we can iterate over it conveniently :)
    let $yesno := (true(), false())
(: Define the major axes so we can iterate over them :)
(: Axis 1:  internal DTD?  external DTD? :)
    for $intdtd in $yesno,
        $extdtd in $yesno,
        (: Subvariation:  if $extdtd, 
           then URI is absolute or relative :)
        $uritype in ('', 'relative', 'absolute'),
        $reltype in ('', 'down', 'up', 'deceptive'),
        $abstype in ('', 'abs', 'diffhost', 'diffscheme'),
(: Axis 2:  valid or invalid :)
        $valid in $yesno
The consistency constraints are also more complex:
(: Specify consistency constraints :)
  where (: only documents with DTDs can be valid or invalid :)
        (($extdtd or $intdtd) or not($valid))
    and (: only documents with external DTDs have URIs
           for their DTDs :)
        (($extdtd and ($uritype)) 
         or (not($extdtd) and not($uritype)) )
    and (: only relative URIs have relative types :) 
        ($uritype eq 'relative' or not($reltype))
    and (: only absolute URIs can have abstype :)
        ($uritype eq 'absolute' or not($abstype))
    and (: one or the other, please :)
        ( ($reltype) or ($abstype) or not($uritype))

Preliminary results

The initial version of the XiBIT includes three test sets. The first set tests a space of variation concerned with document validity and DTD location; its axes are:

  • location of the DTD: internal subset, external subset, both, or none

  • location of the external DTD subset, if any:

    • relative URI pointing down (e.g. lib/test.dtd)

    • relative URI pointing up (e.g. ../lib/test.dtd)

    • relative URI pointing first up then down (e.g. ../tests/lib/test.dtd, for a test case which is itself located in the tests directory)

    • absolute URI with the same host name (etc.) as the test case

    • absolute URI with a different host name[6]

  • validity of the document

The second and third test sets concern the effect of stylesheet location on processing, for XSLT and for CSS stylesheets. The same variations on relative and absolute URIs are tested for stylesheets as are listed above for DTDs.

As may be seen, this initial version of XiBIT tests simple situations and the results are unlikely to surprise readers familiar with developing and deploying Web sites. (But the author confesses to being surprised by some of them, despite having been deploying XML on the Web successfully for several years.)

  • All browsers tested displayed XML documents without regard to the validity of the document. (Most browsers, of course, do not validate XML documents in any case and so are not in a position to make any distinction. Internet Explorer did once issue error messages instead of displaying invalid documents, but this appears not to be true for current versions of IE, at least since IE 7.)

  • All browsers tested displayed XML documents without regard to the location of the DTD. That is, none of them enforced a same-origin policy against the DTD. Since non-validating browsers typically do not read the external DTD subset at all, this may have a certain logic. Only one browser tested (Internet Explorer) issued an error message of the DTD referred to was unavailable.

  • For documents retrieved using HTTP, all browsers tested enforced the same-origin policy on XSLT stylesheets: stylesheets were retrieved and applied when they were pointed to using relative references or using absolute URIs pointing to the host from which the test case was obtained. Stylesheets located on a different host were not retrieved; some browsers (IE, Firefox) gave error messages for those test cases, others simply displayed a blank screen (as shown for the final case in the screen shot above). Stylesheets located in fact on the same host but referred to using a variant name (e.g. example.org instead of www.example.org, or vice versa) were treated (as is usual for the same-origin policy) as coming from a different domain.

  • For documents retrieved from the file system (i.e. using a file:/// URI), some browsers enforced the same-origin policy on XSLT stylesheets and some did not. Some that did enforce a same-origin policy (e.g. Opera) retrieved and applied any stylesheet available in the file system, while others (e.g. Firefox) retrieved only stylesheets located in the same directory as the test case, or in a subdirectory. (Firefox also refused to retrieve documents by following symbolic links, which makes it extremely difficult to use stylesheets located in a common library directory.) Chrome did not apply XSLT stylesheets to any XML document retrieved from the file system.

  • None of the browsers tested enforced a same-origin policy on CSS stylesheets.[7]

Some practical implications for users of XML may be inferred from these facts. Referring to URIs by way of relative references is more likely to work than referring to them by means of absolute URIs (for the browsers tested so far). Users of Firefox and Chrome, however, will find it impossible to view XML documents from the file system in those browsers, if the documents use stylesheets located outside the directory holding the XML document (and thus referred to using URIs of the form ../lib/house-style.xsl).[8]

Further work

XiBIT is still in an early stage of its development. The main work to be done in the immediate future is to make its core set of tests and test results bigger and better: more test cases, exploring more axes of variation, more test results from more versions of browsers on more operating systems, and better tabulation of the raw results.

After a larger core set of tests and test results has been gathered, it should be possible to perform deeper analysis and gain better understanding of the results, resulting in better practical advice for users of XML who would like to publish XML on the Web. A diagnostic tool to test for and warn about interoperability issues will be built to make the information gained by testing practically useful to individual XML publishers.


[ISO/IEC 2381-01] ISO (International Organization for Standardization), IEC (International Electro-technical Commission). ISO/IEC 2382-1:1993 Information technology — Vocabulary — Part 1: Fundamental terms. Geneva: ISO, 1993. (Cited from Wikipedia article on Interoperability at http://en.wikipedia.org/wiki/Interoperability.)

[Milowski 2009] Milowski, R. Alexander. XML in the Browser: the Next Decade. Presented at Balisage: The Markup Conference 2009 (Montréal, Canada, 11-14 August 2009). In Proceedings of Balisage: The Markup Conference 2009. Balisage Series on Markup Technologies, vol. 3 (2009). doi:10.4242/BalisageVol3.Milowski01. doi:10.4242/BalisageVol3.Milowski01.

[Milowski 2011] Milowski, R. Alexander. Efficient XML Processing in Browsers. Presented at XML Prague 2011 (Prague, 26-27 March 2011). Slides on the Web at http://www.xmlprague.cz/2011/presentations/alex-milowski-efficient-xml.pdf.

[OASIS 2001] OASIS XML Conformance Subcommittee. XML 1.0 Test Suite, Second Edition. Working Draft, 15 March 2001. http://www.oasis-open.org/committees/xml-conformance/suite-v1se/xmlconf-20010315.htm. The actual tests are linked from http://www.oasis-open.org/committees/xml-conformance/xml-test-suite.html.

[W3C 2001] World Wide Web Consortium. XSL Working Group. XSL 1.0 Test Suite. [Ed. Max Froumentin.] [Cambridge, Sophia-Antipolis, Tokyo]: W3C, 28 August 2001. http://www.w3.org/Style/XSL/TestSuite/.

[W3C 2008[?]] World Wide Web Consortium. XML Core Working Group. Extensible Markup Language (XML) Conformance Test Suites. [Ed. Henry S. Thompson.] [Cambridge, Sophia-Antipolis, Tokyo]: W3C, [n.d.; 2008?]. http://www.w3.org/XML/Test/.

[W3C 2010] World Wide Web Consortium. XML Core Working Group. Associating Style Sheets with XML documents 1.0 (Second Edition). W3C Proposed Edited Recommendation 09 September 2010. Ed. James Clark (First Edition), Simon Pieters (Second Edition), Henry S. Thompson (Second Edition). [Cambridge, Sophia-Antipolis, Tokyo]: W3C, 2010. http://www.w3.org/TR/2010/PER-xml-stylesheet-20100909/.

[1] In the light of the concern with the user's ability to exchange one imlementation for another, perhaps the area of concern should be called interchangeability testing, but this paper stays with the conventional term interoperability testing.

[2] Actually, even this reduced set of test cases is probably more than is needed: browsers which do not read the DTD at all do not check to see whether the DTD is served from the same origin as the document instance, and in practice it is not necessary to test them for each of the possible forms that a violation of the same-source policy can take. To keep the size of the active test suite within bounds and make it feasible to run at least a core portion of the test suite casually and without a large investment of time, the core part of the test suite really should be restricted to axes of variation which actually exhibit some variation among some defined set of browsers. Tests on which all browsers behave identically produce relatively little information when run; axes of variation that do not distinguish among browsers will be recorded and documented (after all, the axis might become important if some new browser breaks the pattern), but will not be part of the core of XiBIT.

[3] The figure shows stylesheets with various forms of relative-reference URI and absolute URI successfully applied, and a blank screen for the test xsl.abs.diffhost.xml, which has in its XML stylesheet processing instruction a URI for a stylesheet on a different host.

As may be seen, each stylesheet produces different text and has a different color scheme to make it easier to see at a glance which stylesheet applies to which test case. This is less important in a test of different URI forms than in a test investigating which stylesheet is used by a browser when several XML stylesheet instructions are included.

[4] I.e., sets of bindings for the variables $lang, $uritype, $reltype, and $abstype.

[5] This is nonsensical because by definition a relative reference omits the host name and it defaults to the host of the base URI, in this case that of the test case itself.

[6] In principle, the test set should also test the effect of using a different URI scheme (e.g. ftp or https instead of http). The test-case generation code shown above includes diffscheme as a value, but that line of testing has run into technical difficulties.

[7] The level of CSS conformance turned out to vary dramatically among the browsers tested, but no one interested in CSS needs XiBIT to document that fact.

[8] This problem can be avoided by running an HTTP server on one's personal computer and retrieving a document using an http://localhost URI instead of a file: URI, for those willing to set up a Web server on their local machine.