Can an HTML element have multiple ids?

IDs in XHTML and HTML - The basics

XHTML 1.0 types strictly allow only one id attribute per HTML element. HTML5 continues this trend as the structure and functionality of browsers, CSS, and JavaScript rely on unique IDs. An important mention: xml:id can allow multiple IDs but, it's primarily designed for XML-based docs (like XHTML).

Example:

<p id="p1">...</p> <!-- Like a Highlander, there can be only one! -->
<p id="p2">...</p> <!-- IDs are snowflakes, each one should be unique -->


Alternatives for multiple identifiers - the workarounds

Use class or data-* attributes for non-unique identifications. These provide flexibility for multiple tags without compromising the document's integrity.

When styling or manipulating multiple elements with JavaScript, opt for class attributes. When storing custom data or defining unique values for script actions, go for data-* attributes while preserving a unique id for primary identification.

Example:

<p class="highlighted" data-chapter="1">...</p> <!-- Apply CSS or JavaScript manipulations -->
<p class="highlighted" data-chapter="2">...</p> <!-- Store additional custom data -->


Working with IDs - Best practices and the 'gotchas'

Best practices

• Uniformity: Apply consistent patterns throughout your HTML documents to ease maintenance and readability.
• Descriptive IDs: Choose descriptive names for self-documenting your IDs.
• Namespaces: Prefix your IDs to avoid potential clashes when integrating into other systems or widgets.

Pitfalls

• Fragmentation: Non-unique IDs may disrupt page navigation or JavaScript references.
• Styling conflicts: Identical IDs can cause unpredictable conflicts in CSS styling.
• Scripting bugs: Functions like getElementById() return only the first element with the specified ID, leading to potential scripting bugs.

Complex applications

Ensure that server-side generated content (like in PHP, ASP.NET applications) does not inadvertently create duplicate IDs. Similarly, in JavaScript frameworks (React, Vue.js), DOM reference is often better handled using their own virtual DOM mechanisms.

Example:

<!-- ... ASP.NET -->
<asp:Label ID="Label1" runat="server" Text="Name:" />

<!-- ... React -->
<button ref={myRef}>Click me</button> <!-- No ID required when we have refs. Thanks React! -->
