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Given current HTML5 specs that allows creating custom HTML elements (as long as their name contains a dash), and the fact that Web Components seem to be a feature that's here to stay, I'd like to know why is creating your own custom HTML elements frowned upon?
Note, that I'm not asking whether to use Web Components - which are still a moving target, and even with great polyfills like Polymer might not be ready for production yet. I'm asking about creating your own custom HTML tags and styling them, without attaching any JS APIs to them.
Short answer: I haven't heard any very compelling reasons to avoid them.
However, here are some recurring arguments I've heard made:
Doesn't work in old IE (just document.createElement("my-tag"); should fix that).
Global namespace clashes (same applies to class names, and custom elements in general).
CSS selector performance (doh, this is just about the last thing you should worry about).
Separation of functionality, meaning and presentation. This is actually the only argument I've heard that IMHO has any valid basis to it. You're of course better off with semantic HTML (search engines and all that), but if you were going to use a div for it otherwise, I don't see why you couldn't use a custom tag instead.
One of the arguments against custom tags is their implied incompatibility with screen readers. This issue can be resolved with WAI-ARIA attributes.
There exists an issue in IE11, which breaks table layout if a custom element without display property is inserted inside a table cell. Check the plunker code. Therefore, it's the safest to declare all new elements explicitly, for example like so:
new-element {
display: block;
}
According to many recent HTML specs, when we are using custom attributes (meaning any attributes not defined in the spec), we should prefix them with data-. However, I see no reason to have to do this (unless you require perfectly valid HTML, obviously). Pretty much all current browsers correctly ignore custom attributes, meaning no conflicts except with identically-named attributes from others' code, and we can ignore even this with custom prefixes or something similar (as suggested on the AngularJS directive page). What, if any, other benefits are there? This question has been asked before, at least twice, but both are pretty old.
I forget where I read it, but some guide said custom HTML tags need dashes, and single-word tags aren't valid. First of all, why? Second, should we do this, and why (besides being valid)? Would there be any problem with underscores or camelCase, etc.? Also, conflicts with existing elements shouldn't be a problem, if, like with data attributes, you prefix or suffix them, etc. See the Angular directive page again.
I'm sure all these questions have been asked before, but I'm combining them into one. Is that a good idea (quick, someone ask on Meta)?
The data-* attributes have two advantages:
It is a convention meaning other programmers will understand quickly that it is a custom attribute.
You get a DOM Javascript API for free: HTMLElement.dataset. If you use jQuery, it leverages this to populates the keys and values you find with .data().
The reason for the - in custom element names is for two basic reasons:
It is a quick way for the HTML parser to know it is a custom element instead of a standard element.
You don't run into the issue of a new standard element being added with the same name which would cause conflict if you register a custom Javascript prototype for the DOM element.
Should you use your own custom element name? Right now it is so new that don't expect it to be fully supported. Let's say it does work. You have to balance the issue of the extra complexity with the benefit. If you can get away with a classname, then use a classname. But if you need a whole new element with a custom Javascript DOM prototype for the element, then you may have a valid usage of it.
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.
Correct me if I'm mistaken, but AFAIK, unknown HTML tags in markup (i.e. tags not defined in the HTML spec, like say, <foobar>) will eventually be treated as a regular <div> in an HTML 5 browser environment.
I'm thinking: how supportable is this practice? I mean, if I use unknown HTML tags in my markup, what pitfalls can I expect? Will a velociraptor pounce on me within the next few seconds?
The reason I ask is that if these tags defer to <div>, I can potentially use these tags in a more semantic manner than, say, assigning class names that identify modules. Have a look at this article, for example, of a .media class. Now what if instead of writing up that CSS to target .media, I make it target <media> instead? In my opinion, that makes the markup much more readable and maintainable, but I do acknowledge that it's not "correct" HTML.
EDIT
Just to be transparent, I did find this SO question from a few years back. It's been closed as off-topic, but I feel that I have a valid point in my own wording. It's a close duplicate, I admit, but it's from a few years back, so there might have been changes in the general environ of opinions across web developers about the topic.
user1309389 had a very good answer, and I agree with the appeal to the spec. But I disagree with their conclusion, and I think they're wrong about made-up elements leading to "undefined behaviour". I want to propose an alternative way of thinking about it, rooted in how the spec and browsers actually handle made-up elements.
It's 2015, we're on the verge of the CustomElement spec being widely adopted, and polyfills are readily available. Now is a great time to be wondering about "making up your own elements". In the near future, you'll be able to create new elements with your own choice of tag and behaviour in a fully standard and supported way that everyone can love. But until this lands in all browsers, and until the majority of people are using supporting browsers, you can take advantage of the hard work of the Polymer or X-Tags projects to check out the future of custom elements in a nearly-standard and mostly-supported way that quite a few people can love. This is probably the "right thing" to do. But it doesn't answer your question, and frankly I find "just use X" or "don't do X" to be less helpful than "here's how the spec covers this, and here's what browsers do". So, here's what I love.
Against the heartfelt recommendation (and sometimes screaming) of much of the web dev community, I've been working with "made-up" elements for the past year, in all of my production projects, without a polyfill, and I've had no unsolvable issues (yet), and no complaints. How? By relying on the standard behaviour of HTMLUnknownElement, the part of the W3C spec that covers the case of "made-up" elements. If a browser encounters an unrecognized HTML element, there is a well-defined and unambiguous way that it should be handled, and HTMLUnknownElement defines that behaviour, and you can build on top of that. HTMLUnknownElement also has to be powerful and correct enough to "not break the web" when encountering all the tags that are now obsolete, like the <blink> tag. It's not recommended that you use HTMLUnknownElement, but in theory and in practice, there's absolutely no harm in doing so, if you know what you're doing.
So how does HTMLUnknownElement work? It is just an extension of the HTMLElement interface, which is the standard interface underlying all HTML elements. Unlike most other elements however, HTMLUnknownElement doesn't add any special behaviour — you get a raw element, unadorned with any special behaviour nor constraining rules about use. The HTMLDivElement interface works almost exactly the same way, extending HTMLElement and adding almost no additional behaviour. Put simply, making up your own element is almost identical to using a div or span.
What I like about "making-up" elements is the change of mindset. You should use or invent HTML elements based on several factors, ranging from how clear it makes the markup to read, to how the browser and screen readers and search engines parse your code, to how likely your code is to be "correct" by some objective measure. I sparingly use made-up elements, but I use in exactly the way Richard described, to make things more meaningful for the author of the HTML, not just meaningful to a computer service that extracts metadata. When used in a consistent way across a team, there can be a big benefit since made-up elements can concisely express what they're for.
I particularly like using made-up elements to indicate when I will be using JS to define extra behaviour for an element. For instance, if I have an element that will have children added/removed by JS, I will use a made-up element as a clue that this element is subject to special behaviour. By the same token, I don't use a made-up element when a standard element will suffice. You will see <dynamic-list> live happily next to <div> in my code.
Now, about those pesky validators. Yes, using made-up elements isn't "valid" in the sense that it won't pass a "validator". But many commonly used features, patterns, and systems of modern HTML and JS development fail all the W3C validators. The validators aren't the law — the spec is. And the law isn't binding — the implementations in all the browsers are. The utility of validators has been dimishing for years as the flexability of HTML has been increasing, and as browsers have shifted in their relationship to the spec. Validators are great for people who aren't comfortable with HTML and need guidance. But if you're comfortable taking your guidance from the spec and from browser implementations, there's no reason to worry about being flunked by the validator. Certainly, if you follow many of the guidelines offered by Google, Apple, Microsoft, etc, for implementing any experimental features, you'll be working outside the bounds of the validator. This is absolutely an okay thing to do, so long as you're doing it deliberately and you know enough about what you're doing.
Therefore, if you're going to make up your own elements and rely on HTMLUnknownElement, don't just treat it like a wild west. You need to follow a few simple rules.
You have to use a hyphen in the name of your tag. If you do, you are guaranteed to never collide with a future edition of the HTML spec. So never say <wrong>, always say <quite-right>.
Made-up elements can't be self-closing — you have to close them with a closing tag. You can't just say <wrong> or <still-wrong />, you have to say <totally-good></totally-good>.
You have to define a display property for your element in CSS, otherwise the rendering behaviour is undefined.
That's about it. If you do these things, you should be good to use made-up elements in IE9 and up, relying on the safety net of the HTMLUnknownElement. For me, the benefits far, far outweigh the costs, so I've been using this pattern heavily. I run a SaaS site catering to major industrial corporations, and I've had no trouble or complaints thus far. If you have to support older versions of IE, it's wise to stay far away from any "2015" technology or their crude approximations, and stay safely within the well-trodden parts of the spec.
So, in summary, the answer to your question is "yes, if you know what you're doing".
You should always approach HTML as it is defined in its respective specification. "Defining" new tags is a bit of an extreme approach. It might pass a browser check because it implements various failsafes, but there is no guarantee of this. You're entering the land of Undefined Behaviour, at best. Not to mention you will fail validation tests, but you seem to be aware of that.
If you wish to be more semantically expressive in your markup, you can use HTML5 which defines quite a bit of more descriptive tags for describing the structure of your page instead of generic divs which need to be appended ids or classes.
In the end, a short answer: No, it's bad practice, you shouldn't do it and there could be unforeseen problems later on in your development.
No. You will fail validation, you will get random issues cross browser and you WILL be eaten by said dinosaurs. CSS is the answer if you want your page to behave predictably.
Yes We Can.
There is a new spec going on about custom elements/tag - http://w3c.github.io/webcomponents/spec/custom/.
Only issue with this is you have to use js to register your new element
You can read more about this at
https://developers.google.com/web/fundamentals/getting-started/primers/customelements
Rule #1 of browser interoperability is: don't have errors. No matter how many browsers you test in, there are always browsers you can't test, for instance because they don't exist yet.
Also, unknown elements will be treated as <span>, not <div> by most browsers currently.
If it's really source readability(*) you're after, you should look into XML+XSLT.
That way, you can use all the tag names you want, and make them behave in any way you like and you don't have to worry that <media> will be a real element in some future version of HTML.
One good real world example is the element <picture>. If a website ever used <picture> and relied on the notion that this element would have no styles or special content by itself, they are in trouble now!
(*) With XML+XSLT, the readability will be in the XML part, not the XSLT part, obviously.
Generally not recommendable, e.g. IE wont apply css-styles to unknown tags.
All other browsers render unknown tags as inline-Elements (which causes problems with nesting).
I recommend you the following article: http://diveintohtml5.info/ There is a section about unknown tags.
In my case I use a lot of them into my Webkit-powered game GUI system, and everything works.
W3C says:
HTML5 supports unknown tags as inline elements, but it recommends CSS
styling for it.
Here is the source: https://www.w3schools.com/html/html5_browsers.asp
In your example you are talking about <media>, it's could be great but if html6 adds this tag for another element, your code won't be retrocompatible.
The one downside that worries me is what if a custom tag I use now, becomes an official HTML-tag next year or even later?
Therefore how about this: Instead of custom tags use 'div' + a custom CSS-class.
CSS-classes are meant to be custom, it is definitely ok to have your own custom CSS-classes. Your div can then further have any number of CSS-classes associated with it making the semantic machinery even more flexible, you could call it multiple inheritance.
Instead of div you could use span for the same purpose. I would like to use something shorter actually, say p but unfortunately p has its own special behavior of what happens if you don't close it.
But definitely if you go the route of expressing semantics with CSS-classes then it does help to use a tag-name that is as short as possible. I wish there was something shorter than div, say for instance t for 'tag'.
By default, custom elements main parts of many JavaScript-Framwork. It is state of the art in modern javascript.:
var XFoo = document.registerElement('x-foo', {
prototype: Object.create(HTMLElement.prototype)
});
https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements
https://www.html5rocks.com/en/tutorials/webcomponents/customelements/
Is it OK to use unknown HTML tags? If you ask wrong questions, you get allways wrong answers. If you define your unknown tag with javascript as a CustomeElement , your unknown tag is no longer part of the HTML5/HTML-definition.
Yes, you can use unknown tags, if you define the tag in your javascript.
What's wrong with the judicious use of < !-- your stuff here -- >. It worked for scripts back around the time of the Cretaceous–Paleogene boundary. Dinosaurs ceased to be a problem around that time, that is apart from the flying, feathered variety.
It's bad practice and you should not do it. The browser renders it as div as fallback solution in most cases but it's not valid html and therefore never will pass a validity test.
How to fix that
element "SU:BADGE" undefined <su:badge layout="5"></su:badge>
I use HTML 4.01 Transitional on my website ...
The BADGE is from stumbleupon.com website
Thank you . Regards.
Since such markup is not valid in HTML 4.01, the validator is just doing what you asked it to do: reporting any reportable markup errors.
The only practical reason why such error messages might be disturbing is that if there are many of them, you might accidentally miss to notice some other error messages, relating to constructs where your markup unintentionally deviates from HTML 4.01. If this is an issue, consider writing a custom DTD. It requires some understanding of SGML, but so does the use of validators in general, does it not?
On the other hand, you might decide to refrain from using tags suggested by people who cannot do such a simple thing using valid HTML. There are many ways to put information on a web page in a manner that does not affect the visible appearance but can be retrieved by programs that search for specific constructs (e.g., meta tags). They decided on a way that causes problems to authors who wish to use validtors.
You don’t.
Validation is nice (especially as a sanity check while developing) but if you don’t use valid markup then there’s nothing you can do, and if you need invalid markup (because you use a third-party API for instance) then validation simply ceases to be relevant.
Alternatively, you could serve your page as XML instead of HTML 4 and define an appropriate su XML namespace. But since this would mean that some browsers no longer display your page correctly this is more a theoretical possibility.