\n\n\n\nFor achieving cross-browser compatibility and adhering to W3C's Web Components specification, employ WebComponents.js polyfill. Keep up to speed on updates and browser compatibility tables via its GitHub page.","image":"https://explain.codes/media/static/images/eightify-logo.svg","author":{"@type":"Person","name":"Anton Shumikhin","url":"https://explain.codes//author/anton-shumikhin"},"publisher":{"@type":"Organization","name":"Rational Expressions, Inc","logo":{"@type":"ImageObject","url":"https://explain.codes/landing/images/logo@2x.png"}},"datePublished":"2025-01-24T18:15:01.404Z","dateModified":"2025-01-24T18:15:03.016Z"}
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Are custom elements valid HTML5?

html
custom-elements
html5-specification
web-components
Anton ShumikhinbyAnton Shumikhin·Jan 24, 2025
TLDR

Yes, custom elements are valid in HTML5 provided their names contain at least one dash (-). These elements, defined using the customElements.define() method, are part of HTML5's specification. They offer reusability and encapsulation of functionality in a clear, modular manner.

<script> class MyTag extends HTMLElement { // Listen, class! Welcome to the cool world of custom elements! } // Fresher alert - defining custom elements like a pro! customElements.define('my-tag', MyTag); </script> <my-tag></my-tag>

For achieving cross-browser compatibility and adhering to W3C's Web Components specification, employ WebComponents.js polyfill. Keep up to speed on updates and browser compatibility tables via its GitHub page.

Understanding custom elements

Defining autonomous and customized elements

Autonomous custom elements are entirely new; they don't borrow from standard HTML elements. In contrast, customized built-in elements extend the existing HTML syntax and functionality. The key naming convention for these custom elements includes incorporating at least one dash (-) to avert potential clashes with standard HTML elements in future updates.

Placement and styling considerations

Understanding HTML's content model helps ensure custom elements' appropriate integration within your document structure. For styling purposes, CSS is invaluable when dealing with custom tags as they come unstyled by default. Coupling this with JavaScript lets you instill interactive and customized behavior into these elements.

Browser's perspective and validation

Browsers handle custom tags per the HTML5 specification, regardless of whether such tags make sense to HTML validators. For unrecognized or undefined custom element names, the HTMLUnknownElement interface comes to the aid, helping these elements cohabit with others in the DOM.

Attribute naming conventions

While naming custom elements consider prefixing with "x-", custom attributes instead should kick off with "data-" – a practice both valid and standardized across contemporary web development.

Custom elements in real-life scenarios

When developing web applications, the ability to extend the functionality of existing HTML elements can make your job a lot easier. Customized built-in elements are perfect for this. They enable comprehensive behavior customization while preserving the normality and familiarity of standard HTML elements.

Working with Polyfills

Older browsers might be grumpy and obstinate when it comes to understanding custom elements. Fear not, for polyfills like WebComponents.js exist to teach these older browsers some new tricks, ensuring your modern standards get the respect they deserve even in a legacy environment.

Staying informed

In the ever-evolving world of web development, staying up to date can make the difference between writing code that works and code that shines. Regularly visiting resources like the WebComponents.js GitHub helps keep your knowledge fresh in terms of updates and best practices.