Why id Attribute Value must be Unique in HTML?

In web development, using the id attribute is a very basic aspect. You can use the id attribute to identify the elements that you want to style using CSS and manipulate through JavaScript. In this blog, we will discuss the role of the id attribute, its significance, and the best practices.

Table of Contents:

• Understanding the id Attribute
• Why Does the id Attribute Have to be Unique?
  • 1. JavaScript Selectors Depend on Uniqueness
  • 2. CSS Conflicts
  • 3. Accessibility Issues
  • 4. Validation Errors
• What Happens If id Is Not Unique?
• Best Practices for Using id Attributes
  • 1. Always Use Unique ids
  • 2. Use Class for Multiple Elements
  • 3. Follow a Naming Convention
  • 4. Avoid Using ids for Styling Only
  • 5. Use Data Attributes When Needed
• When Is It Acceptable to Have Duplicate ids?
• Conclusion
• Why id Attribute Value must be Unique in HTML? – FAQs

Understanding the id Attribute

The id attribute is like a unique name given to the element. Here’s how you can use it: 

• CSS Styling: It can be used for applying a specific style to the elements. 
• JavaScript Interaction: You can directly access elements with document.getElementById().
• Accessibility: The navigation will be easy, and it also helps the screen readers. 
• Anchor Links: You can use them in the link to jump to the specific parts of the page.

Why Does the id Attribute Have to be Unique?

The id attribute must be unique because the HTML specification does not support the sharing of id among elements. Let’s see different reasons why id attribute has to be unique:

1. JavaScript Selectors Depend on Uniqueness

You can use functions like document.getElementById(‘elementID’) for returning the first matching element. JavaScript also won’t support the sharing of the id attribute. 

2. CSS Conflicts

If the CSS has a duplicate id, it leads to unexpected behaviour in the styling as CSS allows styling by id (e.g., #elementID { color: red; }).

3. Accessibility Issues

For navigation purposes, the id should be unique. It creates a problem with users who depend on screen readers.

4. Validation Errors

The validator flag in the HTML makes the duplicate id an error. This leads to a reduction in the quality of the code. 

What Happens If id Is Not Unique?

If an HTML document contains multiple elements with the same id, the following issues may arise:

• JavaScript Malfunction: The element can be manipulated via getElementById(), but it can only return the first occurrence, and the rest of them are all ignored.
• CSS Confusion: There will be inconsistencies in the applied style, which may lead to issues in the layout. 
• Accessibility Barriers: The User may find difficulties in navigating the page.
• SEO Implications: Search engines might not properly index pages with invalid HTML, potentially affecting rankings.

Best Practices for Using id Attributes

You can follow the best practices below for smooth functionality and maintenance of the HTML code:

1. Always Use Unique ids

To avoid conflicts, the id should be unique. You can use the class whenever you want to apply the same styling property to multiple elements. 

Example: Correct Usage

<div id="header">Website Header</div>

<div id="main-content">Main Content</div>

<div id="footer">Website Footer</div>

Output 

2. Use Class for Multiple Elements

For styling or manipulating multiple elements, you can use a class instead of an id. 

Example:

Incorrect Usage

<div id="box">Box 1</div>

<div id="box">Box 2</div>

Correct Usage

<div class="box">Box 1</div>

<div class="box">Box 2</div>

3. Follow a Naming Convention

Use meaningful and descriptive names for id attributes, such as:

<div id="header-navigation">Navigation</div>

<div id="footer-information">Footer Info</div>

4. Avoid Using ids for Styling Only

Instead of styling elements with ids, prefer class selectors:

Bad Practice

#button { background-color: blue; }

Good Practice

button { background-color: blue; }

Example HTML:

<button class="button">Click Me</button>

5. Use Data Attributes When Needed

If you need to store additional information, consider using data-* attributes instead of id duplication.

<div data-user-id=”123″>User Info</div>

When Is It Acceptable to Have Duplicate ids?

Although the HTML specification discourages duplicate ids, some edge cases might justify their usage:

1. Templating Systems: In dynamically generated content, accidental id duplication can occur. Ensure templates properly generate unique ids.
2. Client-Side Rendering: In JavaScript frameworks like React, Angular, or Vue, temporary duplicate ids might exist before the virtual DOM updates.
3. iframes and Shadow DOM: If elements are inside different iframes or Shadow DOMs, they can have identical ids since they exist in separate document scopes.

Conclusion

To ensure proper functionality, consistency in the styling, and accessibility, the id attribute should be unique within the HTML document. If your code has many duplicate ids, it makes the debugging process harder, and this leads to unexpected behaviour. You can follow the best practices and use classes whenever it is necessary. Therefore, you get a clean HTML code.

Why id Attribute Value must be Unique in HTML? – FAQs