What Happens When You Use Duplicate IDs in HTML?

Multiple Elements with Same ID: A Journey into Invalidity

Despite the clear guidance from W3C that no two elements in a page should share the same ID, a puzzling phenomenon arises when multiple elements possess identical IDs. Let's investigate the implications of this anomaly.

Browser Behavior

Browsers often adopt a "fail silently" approach, interpreting invalid HTML to conform to their expectations. Thus, while it is invalid, the HTML example with two elements having the same ID works in all browsers.

Potential Side Effects

However, by deviating from the specification, you open the door to unforeseen consequences. For instance, using the getElementById() method will only retrieve the first element with the specified ID.

A Safer Alternative

Instead of using multiple elements with the same ID, employ class names for styling groups of elements. This practice adheres to the intended design of CSS selectors.

Querying Elements with the Same ID

If you absolutely must select multiple elements with the same ID, you can resort to attribute selectors:

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

Note that this approach may not work in older browser versions such as IE7 and below.

Conclusion

While browsers may accommodate invalid HTML, it is strongly advised against relying on such behavior. For consistency, maintainability, and to avoid potential issues, use class names for styling similar elements.

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