Can Multiple HTML Elements Share the Same ID and Still Respond to a CSS Selector?

Element IDs and Multiple Responses in CSS Selectors

In HTML, each element should have a unique ID attribute within a document. However, a common question arises: can multiple elements share the same ID value and respond to a single CSS ID selector?

W3C Documentation

According to the W3C documentation, "no two ID attributes can have the same value... an ID attribute can be used to uniquely identify its element." This suggests that using the same ID for multiple elements is not a valid practice.

Browser Behavior

Despite the W3C recommendation, browsers often tolerate invalid HTML and attempt to handle it. For instance, in the following example:

#red {
  color: red;
}

<p>

Both paragraphs will appear in red, even though they share the same ID. This occurs because browsers try to "fail silently" by guessing the developer's intent and interpreting the invalid HTML accordingly.

Risks and Limitations

While browsers may render the page correctly in the above scenario, deviating from the HTML specification can lead to unforeseen side effects. For example, using document.getElementById('red') will only return the first matching element. Additionally, testing across different browsers is crucial to ensure consistency.

Avoid Same IDs for CSS Selectors

As a best practice, avoid using the same ID for multiple elements. Instead, consider using class names to target elements that share similar styles. Class names are specifically designed for this purpose and ensure that each element has a unique identifier.

Alternative for Multiple Elements with Same ID

If selecting multiple elements with the same ID is absolutely necessary, consider using an attribute selector:

document.querySelectorAll('p[id="red"]');

However, this approach is not supported in Internet Explorer 7 and below.

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