I am using angular-ui-select within a website where the styled select fields are configured with an own tag named ui-select. This works great, but doing a W3C validation leads to this error:
Element ui-select not allowed as child of element div in this context. (Suppressing further errors from this subtree.)
Here's an example code:
<!doctype html>
<html lang="en">
<head><title>x</title></head>
<body>
<div>
<ui-select></ui-select>
</div>
</body></html>
I understand that <ui-select> is not expected to be there but how can I handle this better?
Can I wrap it into a different tag or is there a different approach for ui-select instead of using HTML markup?
W3C HTML5 validator maintainer here. The short answer with regard to the validator behavior right now is, the validator's going to emit errors for any custom elements you use in documents, and currently there's no way you as a user can work around it doing that—and it's going to continue that way for some time longer until we get around to figuring out a solution.
We're having some ongoing discussions about how to solve this. Changing the validator to just ignore any element name with a hyphen is not viable as a complete solution, because the consequence of that is we could then not practically check any child elements it might have—we'd just have to ignore the entire subtree, because to do otherwise would lead to other errors. So that's way short of being an ideal solution.
Anyway, I'd love to find a good way to solve this, so if others have ideas I'd like to hear them. Two good places to send ideas/proposals on this are the public-webapps#w3.org mailing list https://lists.w3.org/Archives/Public/public-webapps/ and the whatwg#whatwg.org mailing list https://whatwg.org/mailing-list#specs
One idea I've thought of myself is, we could just have the validator treat all custom elements in the same way it currently treats the <div> element (as far as where it's allowed in a document and what child elements it's allowed to contain). That's also short of ideal, but at least it would give a way to check for errors in descendant elements in the custom element's subtree.
Update 2017-02-06: the W3C HTML Checker now supports custom elements
So, I added support for custom elements to the W3C HTML Checker (validator) on 2016-12-16 and a few days later refined it to do more detailed checking for prohibited names.
The trick I ended up figuring out to implement it in the checker architecture—which is at its core a RelaxNG grammar/schema-based validator—was to add a pre-processing filter that take any elements that have a hyphen in their element name, and puts them in a separate XML namespace.
Then I updated the RelaxNG schema to allow any elements from that XML namespace anywhere. (Which is ironic because I pretty much hate XML namespaces and all the problems they cause.)
So we’re now looking at doing something similar for custom-attribute names—probably just by defining those as being any attribute names that contain a hyphen (like custom-element names).
But the HTML checker can’t be changed to allow custom-attribute names until the HTML spec is updated to allow them. For that, see the proposal being discussed in the HTML-spec issue tracker.
That's indeed a long-known issue with AngularJS.
A few things you can do:
Instead of using the element <ui-select>, you can use <div ui-select>, but that will still fail on the argument.
An argument prefixed with x- or data- will pass but I am not sure ui-select supports that.
HTML W3C validation is useful, but I think mostly important for HTML emails so they don't get screened as spam. It's also good for search engines, but really not that critical.
If you look at 'why validate', the reasons are mostly for cleanliness, ease of debugging, and overall good practice.
Angular (un?)fortunately expands the realm of possibilities for HTML5, in a way that, naturally, deviates from the latest specifications for HTML.
We are having the same problem using Knockout custom components.
http://knockoutjs.com/documentation/component-overview.html
I added a suggestion how to enhance the validator with a minor enhancement for users wanting to use custom elements even if the specification is not yet final (http://w3c.github.io/webcomponents/spec/custom/#custom-tag-example):
https://github.com/validator/validator/issues/94
In my class, I was playing around and found out that CSS works with made-up elements.
Example:
imsocool {
color:blue;
}
<imsocool>HELLO</imsocool>
When my professor first saw me using this, he was a bit surprised that made-up elements worked and recommended I simply change all of my made up elements to paragraphs with ID's.
Why doesn't my professor want me to use made-up elements? They work effectively.
Also, why didn't he know that made-up elements exist and work with CSS. Are they uncommon?
Why does CSS work with fake elements?
(Most) browsers are designed to be (to some degree) forward compatible with future additions to HTML. Unrecognised elements are parsed into the DOM, but have no semantics or specialised default rendering associated with them.
When a new element is added to the specification, sometimes CSS, JavaScript and ARIA can be used to provide the same functionality in older browsers (and the elements have to appear in the DOM for those languages to be able to manipulate them to add that functionality).
(There is a specification for custom elements, but they have specific naming requirements and require registering using JavaScript.)
Why doesn't my professor want me to use made-up elements?
They are not allowed by the HTML specification
They might conflict with future standard elements with the same name
There is probably an existing HTML element that is better suited to the task
Also; why didn't he know that made-up elements existed and worked with CSS. Are they uncommon?
Yes. People don't use them because they have the above problems.
TL;DR
Custom tags are invalid in HTML. This may lead to rendering issues.
Makes future development more difficult since code is not portable.
Valid HTML offers a lot of benefits such as SEO, speed, and professionalism.
Long Answer
There are some arguments that code with custom tags is more usable.
However, it leads to invalid HTML. Which is not good for your site.
The Point of Valid CSS/HTML | StackOverflow
Google prefers it so it is good for SEO.
It makes your web page more likely to work in browsers you haven't tested.
It makes you look more professional (to some developers at least)
Compliant browsers can render [valid HTML faster]
It points out a bunch of obscure bugs you've probably missed that affect things you probably haven't tested e.g. the codepage or language set of the page.
Why Validate | W3C
Validation as a debugging tool
Validation as a future-proof quality check
Validation eases maintenance
Validation helps teach good practices
Validation is a sign of professionalism
YADA (yet another (different) answer)
Edit: Please see the comment from BoltClock below regarding type vs tag vs element. I usually don't worry about semantics but his comment is very appropriate and informative.
Although there are already a bunch of good replies, you indicated that your professor prompted you to post this question so it appears you are (formally) in school. I thought I would expound a little bit more in depth about not only CSS but also the mechanics of web browsers. According to Wikipedia, "CSS is a style sheet language used for describing ... a document written in a markup language." (I added the emphasis on "a") Notice that it doesn't say "written in HTML" much less a specific version of HTML. CSS can be used on HTML, XHTML, XML, SGML, XAML, etc. Of course, you need something that will render each of these document types that will also apply styling. By definition, CSS does not know / understand / care about specific markup language tags. So, the tags may be "invalid" as far as HTML is concerned, but there is no concept of a "valid" tag/element/type in CSS.
Modern visual browsers are not monolithic programs. They are an amalgam of different "engines" that have specific jobs to do. At a bare minimum I can think of 3 engines, the rendering engine, the CSS engine, and the javascript engine/VM. Not sure if the parser is part of the rendering engine (or vice versa) or if it is a separate engine, but you get the idea.
Whether or not a visual browser (others have already addressed the fact that screen readers might have other challenges dealing with invalid tags) applies the formatting depends on whether the parser leaves the "invalid" tag in the document and then whether the rendering engine applies styles to that tag. Since it would make it more difficult to develop/maintain, CSS engines are not written to understand that "This is an HTML document so here are the list of valid tags / elements / types." CSS engines simply find tags / elements / types and then tell the rendering engine, "Here are the styles you should apply." Whether or not the rendering engine decides to actually apply the styles is up it.
Here is an easy way to think of the basic flow from engine to engine: parser -> CSS -> rendering. In reality it is much more convoluted but this is good enough for starters.
This answer is already too long so I will end there.
Unknown elements are treated as divs by modern browsers. That's why they work. This is part of the oncoming HTML5 standard that introduces a modular structure to which new elements can be added.
In older browsers (I think IE7-) you can apply a Javascript-trick after which they will work as well.
Here is a related question I found when looking for an example.
Here is a question about the Javascript fix. Turns out it is indeed IE7 that doesn't support these elements out of the box.
Also; why didn't he know that made-up tags existed and worked with CSS. Are they uncommon?
Yes, quite. But especially: they don't serve additional purpose. And they are new to html5. In earlier versions of HTML an unknown tag was invalid.
Also, teachers seem to have gaps in their knowledge, sometimes. This might be due to the fact that they need to teach students the basics about a given subject, and it doesn't really pay off to know all ins and outs and be really up to date.
I once got detention because a teacher thought I programmed a virus, just because I could make a computer play music using the play command in GWBasic. (True story, and yes, long ago). But whatever the reason, I think the advice not to use custome elements is a sound one.
Actually you can use custom elements. Here is the W3C spec on this subject:
http://w3c.github.io/webcomponents/spec/custom/
And here is a tutorial explaining how to use them:
http://www.html5rocks.com/en/tutorials/webcomponents/customelements/
As pointed out by #Quentin: this is a draft specification in the early days of development, and that it imposes restrictions on what the element names can be.
There are a few things about the other answers that are either just poorly phrased or perhaps a little incorrect.
FALSE(ish): Non-standard HTML elements are "not allowed", "illegal", or "invalid".
Not necessarily. They're "non-conforming". What's the difference? Something can "not conform" and still be "allowed". The W3C aren't going to send the HTML police to your home and haul you away.
The W3C left things this way for a reason. Conformance and specifications are defined by a community. If you happen to have a smaller community consuming HTML for more specific purposes and they all agree on some new Elements they need to make things easier, they can have what the W3C refers to as "other applicable specifications". (this is a gross over simplification, obviously, but you get the idea)
That said, strict validators will declare your non-standard elements to be "invalid". but that's because the validator's job is to ensure conformance to whatever spec it's validating for, not to ensure "legality" for the browser or for use.
FALSE(ish): Non-standard HTML elements will result in rendering issues
Possibly, but unlikely. (replace "will" with "might") The only way this should result in a rendering issue is if your custom element conflicts with another specification, such as a change to the HTML spec or another specification being honored within the same system (such as SVG, Math, or something custom).
In fact, the reason CSS can style non-standard tags is because the HTML specification clearly states that:
User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them
Note: if you want to use a custom tag, just remember a change to the HTML spec at a later time could blow your styling up, so be prepared. It's really unlikely that the W3C will implement the <imsocool> tag, however.
Non-standard tags and JavaScript (via the DOM)
The reason you can access and alter custom elements using JavaScript is because the specification even talks about how they should be handled in the DOM, which is the (really horrible) API that allows you to manipulate the elements on your page.
The HTMLUnknownElement interface must be used for HTML elements that are not defined by this specification (or other applicable specifications).
TL;DR: Conforming to the spec is done for purposes of communication and safety. Non-conformance is still allowed by everything but a validator, whose sole purpose is to enforce conformity, but whose use is optional.
For example:
var wee = document.createElement('wee');
console.log(wee.toString()); //[object HTMLUnknownElement]
(I'm sure this will draw flames, but there's my 2 cents)
According to the specs:
CSS
A type selector is the name of a document language element type written using the syntax of CSS qualified names
I thought this was called the element selector, but apparently it is actually the type selector. The spec goes on to talk about CSS qualified names which put no restriction on what the names actually are. That is to say that as long as the type selector matches CSS qualified name syntax it is technically correct CSS and will match the element in the document. There is no CSS-specific restriction on elements that do not exist in a particular spec -- HTML or otherwise.
HTML
There is no official restriction on including any tags in the document that you want. However, the documentation does say
Authors must not use elements, attributes, or attribute values for purposes other than their appropriate intended semantic purpose, as doing so prevents software from correctly processing the page.
And it later says
Authors must not use elements, attributes, or attribute values that are not permitted by this specification or other applicable specifications, as doing so makes it significantly harder for the language to be extended in the future.
I'm not sure specifically where or if the spec says that unkown elements are allowed, but it does talk about the HTMLUnknownElement interface for unrecognized elements. Some browsers may not even recognize elements that are in the current spec (IE8 comes to mind).
There is a draft for custom elements, though, but I doubt it is implemented anywhere yet.
This is possible with html5 but you need to take into consideration of older browsers.
If you do decide to use them then, make sure to COMMENT your html!! Some people may have some trouble figuring out what it is so a comment could save them a ton of time.
Something like this,
<!-- Custom tags in use, refer to their CSS for aid -->
When you make your own custom tag/elements the older browsers will have no clue what that is just like html5 elements like nav/section.
If you are interested in this concept then I recommend to do it the right way.
Getting started
Custom Elements allow web developers to define new types of HTML
elements. The spec is one of several new API primitives landing under
the Web Components umbrella, but it's quite possibly the most
important. Web Components don't exist without the features unlocked by
custom elements:
Define new HTML/DOM elements Create elements that extend from other
elements Logically bundle together custom functionality into a single
tag Extend the API of existing DOM elements
There is a lot you can do with it and it does make your script beautiful as this article likes to put it. Custom Elements defining new elements in HTML.
So lets recap,
Pros
Very elegant and easy to read.
It is nice to not see so many divs. :p
Allows a unique feel to the code
Cons
Older browser support is a strong thing to consider.
Other developers may have no clue what to do if they don't know about custom tags. (Explain to them or add comments to inform them)
Lastly one thing to take into consideration, but I am unsure, is block and inline elements. By using custom tags you are going to end up writing more css because of the custom tag won't have a default side to it.
The choice is entirely up to you and you should base it on what the project is asking for.
Update 1/2/2014
Here is a very helpful article I found and figured I would share, Custom Elements.
Learn the tech Why Custom Elements? Custom Elements let authors define
their own elements. Authors associate JavaScript code with custom tag
names, and then use those custom tag names as they would any standard
tag.
For example, after registering a special kind of button called
super-button, use the super button just like this:
Custom elements are still elements. We
can create, use, manipulate, and compose them just as easily as any
standard or today.
This seems like a very good library to use but I did notice it didn't pass Window's Build status. This is also in a pre-alpha I believe so I would keep an eye on this while it develops.
Why doesn't he want you to use them? They are not common nor part of the HTML5 standard.
Technically, they are not allowed. They are a hack.
I like them myself, though. You may be interested in XHTML5. It allows you to define your own tags and use them as part of the standard.
Also, as others have pointed out, they are invalid and thus not portable.
Why didn't he know that they exist? I don't know, except that they are not common. Possibly he was just not aware that you could.
Made-up tags are hardly ever used, because it's unlikely that they will work reliably in every current browser, and every future browser.
A browser has to parse the HTML code into elements that it knows, to made-up tags will be converted into something else to fit in the document object model (DOM). As the web standards doesn't cover how to handle everyting that is outside of the standards, web browsers tend to handle non-standars code in different ways.
Web development is tricky enough with a bunch of different browsers that have their own quirks, without adding another element of uncertainty. The best bet it to stick with things that are actually in the standards, that is what the browser vendors try to follow, so that has the best chance to actually work.
I think made-up tags are just potentially more confusing or unclear than p's with IDs (some block of text generally). We all know a p with an ID is a paragraph, but who knows what made-up tags are intended for? At least that's my thought. :) Therefore this is more of a style / clarity issue than one of functionality.
Others have made excellent points but its worth noting that if you look at a framework such as AngularJS, there is a very valid case for custom elements and attributes. These convey not only better semantic meaning to the xml, but they also can provide behavior, look and feel for the web page.
CSS is a style sheet language that can be used to present XML documents, not only (X)HTML documents. Your snippet with the made-up tags could be part of a legal XML document; it would be one if you enclose it in a single root element. Probably you already have a <html> ...</html> around it? Any current browser can display XML documents.
Of course it is not a very good XML document, it lacks a grammar and an XML declaration. If you use an HTML declaration header instead (and probably a server configuration that sends the correct mime type) it would instead be illegal HTML.
(X)HTML has advantages over plain XML as elements have a semantic meaning that is useful in the context of a web page presentation. Tools can work with this semantics, other developers know the meaning, it is less error prone and better to read.
But in other contexts it is better to use CSS with XML and/or XSLT to do the presentation. This is what you did. As this wasn't your task, you didn't know what you were doing, and HTML/CSS is the better way to go most of the time you should stick to it in your scenario.
You should add an (X)HTML header to your document so tools can give you meaningful error messages.
...I simply change all of my made up tags to paragraphs with ID's.
I actually take issue with his suggestion of how to do it properly.
A <p> tag is for paragraphs. I see people using it all the time instead of a div -- simply for spacing purposes or because it seems gentler. If it's not a paragraph, don't use it.
You don't need or want to stick ID's on everything unless you need to target it specifically (e.g. with Javascript). Use classes or just a straight-up div.
From its early days CSS was designed to be markup agnostic so it can be used with any markup language producing tree alike DOM structures (SVG for example). Any tag that comply to name token production is perfectly valid in CSS. So your question is rather about HTML than CSS itself.
Elements with custom tags are supported by HTML5 specification. HTML5 standardize the way how unknown elements must be parsed in the DOM. So HTML5 is the first HTML specification that enables custom elements strictly speaking. You just need to use HTML5 doctype <!DOCTYPE html> in your document.
As of custom tag names themselves...
This document http://www.w3.org/TR/custom-elements/ recommends custom tags you choose to contain at least one '-' (dash) symbol. This way they will not conflict with future HTML elements. Therefore you'd better change your doc to something like this:
<style>
so-cool {
color:blue;
}
</style>
<body>
<so-cool>HELLO</so-cool>
</body>
Surprisingly, nobody (including my past self) mentioned accessibility. Another reason that using valid tags instead of custom ones is for compatibility with the greatest amount of software, including screen-readers and other tools that people need for accessibility purposes. Moreover, accessibility laws like WAI require making accessible websites, which generally means requiring them to use valid markup.
Apparently nobody mentioned it, so I will.
This is a by-product of browser wars.
Back in the 1990’s when the Internet was first starting to go mainstream, competition incrased in the browser market. To stay competitive and draw users, some browsers (most notably Internet Explorer) tried to be helpful and “user-friendly” by attempting to figure out what page designers meant and thus allowed markup that are incorrect (e.g., <b><i>foobar</b></i> would correctly render as bold-italics).
This made sense to some degree because if one browser kept complaining about syntax errors while another ate anything you threw at it and spit out a (more-or-less) correct result, then people would naturally flock to the latter.
While many thought the browser wars were over, a new war between browser vendors has reignited in the past few years since Chrome was released, Apple started growing again and pushing Safari, and IE lost its dominance. (You could call it a “cold war” due to the perceived cooperation and support of standards by browser vendors.) Therefore, it is not a surprise that even contemporary browsers which supposedly conform strictly to web standards actually try to be “clever” and allow standard-breaking behavior such as this in order to try to gain an advantage as before.
Unfortunately, this permissive behavior led to a massive (some might even say cancerous) growth of poorly marked up webpages. Because IE was the most lenient and popular browser, and due to Microsoft’s continued flouting of standards, IE became infamous for encouraging and promoting bad design and propagating and perpetuating broken pages.
You may be able to get away with using quirks and exploits like that on some browsers for now, but other than the occasional puzzle or game or something, you should always stick to web standards when creating web pages and sites to ensure they display correctly and avoid them becoming broken (possibly completely ignored) with a browser update.
While browsers will generally relate CSS to HTML tags regardless of whether or not they are valid, you should ABSOLUTELY NOT do this.
There is technically nothing wrong with this from a CSS perspective. However, using made up tags is something you should NEVER do in HTML.
HTML is a markup language, which means that each tag corresponds to a specific type of information.
Your made up tags don't correspond to any type of information. This will create problems from web crawlers, such as Google.
Read more information on the importance of correct markup.
Edit
Divs refer to groups of multiple related elements, meant to be displayed in block form and can be manipulated as such.
Spans refer to elements that are to be styled differenly than the context they are currently in and are meant to be displayed inline, not as a block. An example is if a few words in a sentence needs to be all caps.
Custom tags do not correlate to any standards and thus span/div should be used with class/ID properties instead.
There are very specific exemptions to this, such as Angular JS
Although CSS has a thing called a "tag selector," it doesn't actually know what a tag is. That's left for the document's language to define. CSS was designed to be used not just with HTML, but also with XML, where (assuming you're not using a DTD or other validation scheme) the tags can be just about anything. You could use it with other languages too, though you would need to come up with your own semantics for exactly what things like "tags" and "attributes" correspond to.
Browsers generally apply CSS to unknown tags in HTML, because this is considered better than breaking completely: at least they can display something. But it is very bad practice to use "fake" tags deliberately. One reason for this is that new tags do get defined from time to time, and if one is defined that looks sort of like your fake tag but doesn't quite work the same way, that can cause problems with your site on new browsers.
Why does CSS work with fake elements? Because it doesn't hurt anyone because you're not supposed to use them anyways.
Why doesn't my professor want me to use made-up elements? Because if that element is defined by a specification in the future your element will have an unpredictable behavior.
Also, why didn't he know that made-up elements exist and work with CSS. Are they uncommon? Because he, like most other web developers, understand that we shouldn't use things that might break randomly in the future.
If there is no DTD or schema to validate the H5 document against, how are we supposed to do document validation? And by document validation, I mean "how are we supposed to ensure our html5 documents are both syntactically accurate and structurally sound?" Please help! This is going to become a huge problem for our industry if we have no way to accurately validate HTML5 documents!
Sure, the W3C has an online tool that validates individual pages. But, if I'm creating A LOT of pages (hundreds, say) and I want to validate them in a sort of batch mode, what is the accepted method of ensuring valid structure and syntax? I mean, it seems rather rudimentary to just look at the document and say "yep. that's a valid xml document." What about custom tags? What about tag attributes? It seems like the W3C is leaving us out in the cold a little bit here.
Maybe the best answer will be found in the HTML editor. But then you get DTD/schema fragmentation. Each editor vendor coming up with their own rendition of what a valid structure is.
Maybe the answer is "wait for HTML5 to become official". But I really can't wait for that. I need to start creating and validating content now. I have applications I want to publish that can only be accomplished with html5.
So, any thoughts?
If there is no DTD or schema to validate the H5 document against, how are we supposed to do document validation?
With a specialized HTML5 validator rather then a generic SGML or XML validator.
Obviously, as the specification is still in draft form, the tools that do exist are immature and likely to be out of date or become out of date.
Sure, the W3C has an online tool that validates individual pages. But, if I'm creating A LOT of pages (hundreds, say) and I want to validate them in a sort of batch mode, what is the accepted method of ensuring valid structure and syntax?
Either use a different tool or download the W3C validator and run a local copy. It has a SOAP API so writing a batch validation tool isn't difficult.
What about custom tags?
HTML5 doesn't allow custom elements.
What about tag attributes?
The only custom attributes in HTML5 are data-* attributes, so an HTML 5 validator can recognize them.
It seems like the W3C is leaving us out in the cold a little bit here.
It seems like you expect the state of QA tools for HTML 5 (unfinished) to be up to the same standard as those for HTML 4 (over a decade old). This isn't a realistic expectation.
Maybe the best answer will be found in the HTML editor. But then you get DTD/schema fragmentation. Each editor vendor coming up with their own rendition of what a valid structure is.
The specification is clear (although in flux) even if it isn't expressed in the form of a DTD or schema. If each editor has a different idea of what is valid, then most or all of them are going to be either out of date or just buggy.
Maybe the answer is "wait for HTML5 to become official". But I really can't wait for that. I need to start creating and validating content now. I have applications I want to publish that can only be accomplished with html5.
If you need to live in the bleeding edge, then you have to accept the limitations and risks of doing so.
You might find this question/answer interesting: Will HTML 5 validation be worth the candle? . The answer is written by the developer of http://about.validator.nu/ .
You should start by taking a look at http://about.validator.nu/ .
Some, though not all, of your concerns are addressed there. You can host your own validator, there's a python based submission script, you can use a RESTFUL web service API and there are ways to get validation output in a variety of different forms.
I can't however see a simple way to integrate XHTML5 with other applications of XML such that one can easily create a validator of such compound documents. Not that there's really been a way to do that with earlier versions of XHTML either though.
This is working well for me: https://github.com/hober/html5-el
To get this to work, I renamed the default '/etc/schema/schemas.xml' file in order to move it out of the way and let the 'html5-el' one be used by nxml-mode.
If there is no DTD or schema to validate the H5 document against, how are we supposed to do document validation? And by document validation, I mean "how are we supposed to ensure our html5 documents are both syntactically accurate and structurally sound?" Please help! This is going to become a huge problem for our industry if we have no way to accurately validate HTML5 documents!
If testing pages with either Firefox or Opera, both of those will report errors such as code that is not "well-formed" and mismatched tags. Beyond that, one of the validators such as validator.w3.org or validator.nu will definitely help.
Sure, the W3C has an online tool that validates individual pages. But, if I'm creating A LOT of pages (hundreds, say) and I want to validate them in a sort of batch mode, what is the accepted method of ensuring valid structure and syntax? I mean, it seems rather rudimentary to just look at the document and say "yep. that's a valid xml document."
There are ways to run the W3C validator in batch mode.
What about custom tags? What about tag attributes? It seems like the W3C is leaving us out in the cold a little bit here.
The easy answer to that one is that "custom tags" are simply not considered valid. The Working Group has thoroughly addressed the issue of "distributed extensibility", particularly with respect to allowing "decentralized
parties to create their own languages" and "extension attributes" (http:// lists.w3.org/Archives/Public/public-html/2011Feb/0085.html). There are numerous ways to extend HTML (http://www.whatwg.org/specs/web-apps/current-work/multipage/infrastructure.html#extensibility) but adding custom tags is not one of them. Custom data and microdata attributes should validate fine.
Maybe the answer is "wait for HTML5 to become official". But I really can't wait for that. I need to start creating and validating content now. I have applications I want to publish that can only be accomplished with html5.
Since HTML 5 was stabilized at the end of last year (Dec. 2010), IMO we don't need to wait for it to become an official "recommendation" by the W3C. The stabilized spec provides a solid base that all browser vendors can implement consistently and for the ongoing evolution beyond HTML 5 of the spec, which is now being called the "HTML Living Standard" (Jan. 2011 and later). There is a good diagram of this at http://www.HTML-5.com/html-versions-and-history.html#html-versions (scroll down to see the diagram).
Ok, don't get me wrong, I absolutely love the idea of web standards... wrote and validated a number of pages with strict XHTML 1.0 - however, the web is evolving... and the more I use XML, realize the capacity of the DOM, and realize most browsers don't care one way or the other, the more I realize I realize I just want to break conventions and start using custom attributes.
Example of this is on my current site which uses a "message" attribute on a number of elements, and jQuery to then match those element types and update a footer message (something like a static tooltip). Problem of course is... this isn't actually supported.
My question then is simply is there something of a broader spectrumed doctype that would allow me to use 99% of the XHTML and/or HTML5 standard but throw in some custom attributes?
Or do I just continue to break validation and say to hell with it cause the browser and javascript will "get it" anyway?
The nature of DTDs and XML validation requires that a custom DTD be used if you're adding extra namespaces to a document. See the A List Apart articles Validating a Custom DTD and More About Custom DTDs for details on how to create a custom DTD. I don't know if this is possible within the confines of DTD syntax, but you could consider creating your own namespace and simply declaring "this namespace may contain anything" -- that should provide a nice dumping-ground for custom data without interfering with XHTML parsing.
If you're interested in HTML5, then make sure the names of your custom attributes start with "data-" and they'll validate in an HTML5 validator.
Otherwise, I'd just break validation. XHTML 1.x validation (which is doctype based) and browser interpretation of the markup (which is content-type based) are far enough apart to make XHTML validation of dubious value, once you know what you're doing.