Are custom elements valid HTML5?

I've been unable to find a definitive answer to whether custom tags are valid in HTML5, like this:

<greeting>Hello!</greeting>

I've found nothing in the spec one way or the other:

http://dev.w3.org/html5/spec/single-page.html

And custom tags don't seem to validate with the W3C validator.

The Custom Elements specification is available in Chrome and Opera, and becoming available in other browsers. It provides a means to register custom elements in a formal manner.

> Custom elements are new types of DOM elements that can be defined by > authors. Unlike decorators, which are stateless and ephemeral, custom > elements can encapsulate state and provide script interfaces.

Custom elements is a part of a larger W3 specification called Web Components, along with Templates, HTML Imports, and Shadow DOM.

> Web Components enable Web application authors to define widgets with a > level of visual richness and interactivity not possible with CSS > alone, and ease of composition and reuse not possible with script > libraries today.

However, from this excellent walk through article on Google Developers about Custom Elements v1:

> The name of a custom element must contain a dash (-). So <x-tags>, <my-element>, and <my-awesome-app> are all valid names, while <tabs> and <foo_bar> are not. This requirement is so the HTML parser can distinguish custom elements from regular elements. It also ensures forward compatibility when new tags are added to HTML.

Some Resources

• Example Web Components are available at https://WebComponents.org
• WebComponents.js serves as a polyfill for Web Components until they are supported everywhere. See also the WebComponents.js github page & web browser support table.

It's possible and allowed:

> 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.

http://www.w3.org/TR/html5/infrastructure.html#extensibility-0

But, if you intend to add interactivity, you'll need to make your document invalid (but still fully functional) to accomodate IE's 7 and 8.

See http://blog.svidgen.com/2012/10/building-custom-xhtml5-tags.html (my blog)

N.B. The answer below was correct when it was written in 2012. Since then, things have moved on a bit. The HTML spec now defines two types of custom elements - "autonomous custom elements" and "customized built-in elements". The former can go anywhere phrasing content is expected; which is most places inside body, but not e.g. children of ul or ol elements, or in table elements other than td, th or caption elements. The latter can go where-ever the element that they extend can go.

---

This is actually a consequence of the accumulation of the content model of the elements.

For example, the root element must be an html element.

The html element may only contain A head element followed by a body element.

The body element may only contain Flow content where flow content is defined as the elements: a, abbr, address, area (if it is a descendant of a map element), article, aside, audio, b, bdi, bdo, blockquote, br, button, canvas, cite, code, command, datalist, del, details, dfn, div dl, em, embed, fieldset, figure, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, i, iframe, img, input, ins, kbd, keygen, label, map, mark, math, menu, meter, nav, noscript, object, ol, output, p, pre, progress, q, ruby, s, samp, script, section, select, small, span, strong, style (if the scoped attribute is present), sub, sup, svg, table, textarea, time, u, ul, var, video, wbr and Text

and so on.

At no point does the content model say "you can put any elements you like in this one", which would be necessary for custom elements/tags.

Basic Custom Elements and Attributes

Custom elements and attributes are valid in HTML, provided that:

• Element names are lowercase and begin with x-
• Attribute names are lowercase and begin with data-

For example, <x-foo data-bar="gaz"/> or <br data-bar="gaz"/>.

A common convention for elements is x-foo; x-vendor-feature is recommended.

This handles most cases, since it's arguably rare that a developer would need all the power that comes with registering their elements. The syntax is also adequately valid and stable. A more detailed explanation is below.

Advanced Custom Elements and Attributes

As of 2014, there's a new, much-improved way to register custom elements and attributes. It won't work in older browsers such as IE 9 or Chrome/Firefox 20. But it allows you to use the standard HTMLElement interface, prevent collisions, use non-x-* and non-data-* names, and define custom behavior and syntax for the browser to respect. It requires a bit of fancy JavaScript, as detailed in the links below.

HTML5 Rocks - Defining New Elements in HTML
WebComponents.org - Introduction to Custom Elements
W3C - Web Components: Custom Elements

Regarding The Validity of The Basic Syntax

Using data-* for custom attribute names has been perfectly valid for some time, and even works with older versions of HTML.

W3C - HTML5: Extensibility

As for custom (unregistered) element names, the W3C strongly recommends against them, and considers them non-conforming. But browsers are required to support them, and x-* identifiers won't conflict with future HTML specs and x-vendor-feature identifiers won't conflict with other developers. A custom DTD can be used to work around any picky browsers.

Here are some relevant excerpts from the official docs:

> "Applicable specifications MAY define new document content (e.g. a > foobar element) [...]. If the syntax and semantics of a given > conforming HTML5 document is unchanged by the use of applicable > specification(s), then that document remains a conforming HTML5 > document." > > "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." > > "User agents are not free to handle non-conformant documents as they > please; the processing model described in this specification applies > to implementations regardless of the conformity of the input > documents." > > "The HTMLUnknownElement interface must be used for HTML elements that > are not defined by this specification."

W3C - HTML5: Conforming Documents
WhatWG - HTML Standard: DOM Elements

I would like to point out that the word "valid" can have two different meanings in this context, either of which is potentially, um, valid.

1. Should an HTML document with custom tags be considered valid HTML5? The answer to this is clearly "no." The spec lists exactly what tags are valid in what contexts. This is why an HTML validator will not accept a document with custom tags, or with standard tags in the wrong places (like an "img" tag in the header).

2. Will an HTML document with custom tags be parsed and rendered in a standard, clearly-defined way across browsers? Here, perhaps surprisingly, the answer is "yes." Even though the document would not technically be considered valid HTML5, the HTML5 spec does specify exactly what browsers are supposed to do when they see a custom tag: in short, the custom tag acts kind of like a <span> - it means nothing and does nothing by default, but it can be styled by HTML and accessed by javascript.

To give an updated answer reflecting modern pages.

Custom tags are valid if either,

1. They contain a dash

2. They are embedded XML

   http://example.com/customNamespace">Hello!</greeting>

This assumes you are using the HTML5 doctype <!doctype html>

Considering these simple restrictions it now makes sense to do your best to keep your HTML markup valid (please stop closing tags like <img> and <hr>, it's silly and incorrect unless you use an XHTML doctype, which you probably have no need for).

Given that HTML5 clearly defines the parsing rules a compliant browser will be able to handle any tag that you throw at it even if it isn't strictly valid.

Custom HTML elements are an emerging W3 standard I have been contributing to that enables you to declare and register custom elements with the parser, you can read the spec here: W3 Web Components Custom Elements spec. Additionally, Microsoft supports a library (written by former Mozilla devs), called X-Tag - it makes working with Web Components even easier.

Quoting from the Extensibility section of the HTML5 specification:

> For markup-level features that can be limited to the XML serialization and need not be supported in the HTML serialization, vendors should use the namespace mechanism to define custom namespaces in which the non-standard elements and attributes are supported.

So if you're using the XML serialization of HTML5, its legal for you to do something like this:

<greeting xmlns="http://example.com/customNamespace">Hello!</greeting>

However, if you're using the HTML syntax you are much more limited in what you can do.

> For markup-level features that are intended for use with the HTML syntax, extensions should be limited to new attributes of the form "x-vendor-feature" [...] New element names should not be created.

But those instructions are primarily directed at browser vendors, who would assumedly be providing visual styling and functionality for whatever custom elements they chose to create.

For an author, though, while it may be legal to embed a custom element in the page (at least in the XML serialization), you're not going to get anything more than a node in the DOM. If you want your custom element to actually do something, or be rendered in some special way, you should be looking at the Custom Elements specification.

For a more gentle primer on the subject, read the Web Components Introduction, which also includes information about the Shadow DOM and other related specifications. These specs are still working drafts at the moment - you can see the current status here - but they are being actively developed.

As an example, a simple definition for a greeting element might look something like this:

<element name="greeting">
  <template>
    <style scoped>
      span { color:gray; }
    </style>
    <span>Simon says:</span>
    <q><content/></q>
  </template>
</element>

This tells the browser to render the element content in quotes, and prefixed by the text "Simon says:" which is styled with the color gray. Typically a custom element definition like this would be stored in a separate html file that you would import with a link.

<link rel="import" href="greeting-definition.html" />

Although you can also include it inline if you want.

I've created a working demonstration of the above definition using the Polymer polyfill library which you can see here. Note that this is using an old version of the Polymer library - more recent versions work quite differently. However, with the spec still in development, this is not something I would recommend using in production code anyway.

just use whatever you want without any dom declaration

<container>content here</container>

add your own style (display:block) and it will work with any modern browser

data-* attributes are valid in HTML5 and even in HTML4 all web browsers used to respect them. Adding new tags is technically okay, but is not recommended just because:

1. It may conflict with something added in the future, and
2. Makes the HTML document invalid unless dynamically added via JavaScript.

I use custom tags only in places that Google does not care, for ecample in a game engine iframe, i made a <log> tag that contained <msg>, <error> and <warning> - but through JavaScript only. And it was fully valid, according to the validator. It even works in Internet explorer with its styling! ;]

Custom tags are not valid in HTML5. But currently browsers are supporting to parse them and also you can use them using css. So if you want to use custom tags for current browsers then you can. But the support may be taken away once the browsers implement W3C standards strictly for parsing the HTML content.

I know this question is old, but I have been studying this subject and though some of the above statements are correct, they aren't the only way to create custom elements. For example:

<button id="find">click me</button>
<Query? ?attach="find" ?content="alert( find() );" ?prov="Hello there :D" >
I won't be displayed :D
</Query?>

<style type="text/css">

[\?content] {

display: none;

}

</style>

<script type="text/javascript">

S = document.getElementsByTagName("Query?")[0];

Q = S.getAttribute("?content");

A = document.getElementById( S.getAttribute("?attach") );

function find() {

  return S.getAttribute("?prov");

}

(function() {

A.setAttribute("onclick", Q);

})();

</script>

would work perfectly fine ( in newer versions of Google Chrome, IE, FireFox, and mobile Safari so far ). All you need is just an alpha character (a-z, A-Z) to start the tag, and then you can use any of the non alpha characters after. If in CSS, you must use the "" (backslash) in order to find the element, such as would need Query^ { ... } . But in JS, you just call it how you see it. I hope this helps out. See example here

-Mink CBOS