Declarative AJAX Components and XML Namespaces 
October 16th, 2006
Being on the OpenAjax committee has been pretty interesting so far despite the fact that I have only managed to be in on one call - luckily all the minutes are on the wiki and I have been following that pretty intently.
The declarative markup comittee is looking at how to standardize the definition and instantiation of declarative AJAX components or widgets. A good example of a complex component is the Nitobi Ajax Grid component while the Dojo framework has several smaller widgets like the very cool fisheye menu. To create an Ajax widget in a web page declaratively one might have some markup like this:
Similarly, one could use something like this:
This is very similar to the idea of microformats - and even makes them more expressive. In either case the OpenAjax bootstrapper would start using something like the
Unfortunately, IMHO, people generally opt for the later declaration that uses the build in HTML elements and simply adds custom attributes. While both are nearly identical, the use of namespace prefixes on the tags is possibly more “standard” and ensures that one doesn’t get a Grid component mixed up with a regular DIV element. For the most part, I think the gravitation towards the later declaration is due to the general contempt people hold for Internet Explorer - and indeed most of the people on the declarative markup committee feel this way. Having been building declarative components for Internet Explorer for over six years now we are pretty well versed in the problems with Internet Explorer, and I can assure you that the support for XML namespaces and custom tags in Internet Explorer is perfectly fine. I would argue, as you will see in a moment, that the support is even better in Internet Explorer than in Mozilla. Some Ajax frameworks, such as Dojo, support XML namespaces in Mozilla but not in IE due to perceived deficiencies.
From our experience, Firefox and not Internet Explorer has been the browser that breaks when custom namespaced tags are used. The biggest hurdle is that, as noted in the Dojo manual page that, one “must” (emphasis on the part of the author of the Dojo manual) define the xmlns:mynamespace on the page. Usually that is done on the HTML tag. Anyone that has worked with XML will surely know that if you want to have some elements in a different namespace then declaring that namespace is a fact of life. On the other hand, Firefox works in a quirky way if you define custom XML namespaced tags - forgetting the fact that it doesn’t even care if you define the namespace or not, which seems very bizarre to anyone familiar with XML. For the straight dope on custom tags in IE everyone should check out the relevant MSDN article (it is amazing all the good stuff on MSDN that many people ignore). The jist of the article is that you just need to specify your XML namespace on the HTML element like this:
How completely unexpected! However, it can also be specified on any other element that is a parent of an element that uses the namespace. The article describes the fact that support for custom tags was introduced for enabling behaviours - ie defining custom HTML elements that behave in some custom way - which were a great idea. Admitedly, the one drawback of the IE model is that it does not support the
Having said all this, one should also remember that XHTML is not really there yet.
At any rate, we still want to use XML namespaced tags and there are a few problems we have found with Firefox regarding support for custom XML namespaced elements. The two main problems we have observed are:
- Styles are not applied correctly
- DOM methods do not work correctly
- Self closing tags do not work
To illustrate this, I have made a simple example with some sample markup that might be used in some sort of windowing or panelling situation. I defined some buttons, panels and a title with some interspersed native HTML elements.
<div id="div1">
<ntb:button id="button1">button 1</ntb:button>
<ntb:button id="button2">button 2</ntb:button>
<ntb:panel id="panel1">
<ntb:title id="title1">
<div id="div2">panel title 1</div>
</ntb:title>
<ntb:contents id="contents1">
<div id="div3">Contents div3</div>
<div id="div4">Contents div4</div>
</ntb:contents>
</ntb:panel>
</div>
One common thing to do is define some custom styles for these elements such as this:
ntb\:panel {border:1px solid red;display:block;}
ntb\:contents {color:red;display:block;}
ntb\:title {border:1px solid black;background-color:#3366FF;display:block;}
ntb\:button {border:1px solid black;background-color:#CCCCCC;}
The results are fairly good in IE:
and pretty poor in Mozilla:
Granted that is quite contrived and the likelyhood of specifying styles for the custom elements is pretty low.
This brings us to the second point which is a bit more important. The native DOM methods in Mozilla don’t actually recognize the custom elements; or rather, to be more precise, DOM methods don’t reflect the true DOM hierarchy as it appears in your HTML. As an example, we will try and access the parent node of each of the nodes in our sample custom namespaced HTML. The results were as follows:
IE parentNode()
| Target Node ID | Expected Parent ID | Actual Parent ID |
| title1 | title1 | title1 |
| contents1 | contents1 | contents1 |
| … | … | … |
| ok you get the idea | ||
Mozilla parentNode()
| Target Node ID | Expected Parent ID | Actual Parent ID |
| div2 | title1 | div1 |
| div3 | contents1 | div1 |
| div4 | contents1 | div1 |
| button1 | div1 | div1 |
| button2 | div1 | div1 |
| panel1 | div1 | div1 |
| title1 | panel1 | panel1 |
| contents1 | panel1 | div1 |
Internet Explorer gets the parentNode as one would expect but Mozilla seems to have some difficulty. There are similar difficulties with many other of the DOM methods in Mozilla.
The big problem we have found is that Mozilla does not support self closing tags. One would expect that the following would be equivalent:
<ntb:grid></ntb:grid>
Not so in Mozilla. The later syntax is ok, whereas the self closing tag does not get parsed correctly when you look at the innerHTML of an element and it is even worse once you have self closing tags in conjunction with DOM methods.
Mozilla has to get its act together with custom namespaced tags for declarative AJAX components to ever get anywhere. If anyone wants to compare war stories then please leave some comments.
Technorati tags:ajax, microformat, declarative, components, xml
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